diff --git a/icu4c/source/common/unicode/simpleformatter.h b/icu4c/source/common/unicode/simpleformatter.h
index 7f58106fadb..33bdd6d48fc 100644
--- a/icu4c/source/common/unicode/simpleformatter.h
+++ b/icu4c/source/common/unicode/simpleformatter.h
@@ -25,11 +25,9 @@
 U_NAMESPACE_BEGIN
 
 // Forward declaration:
-namespace number {
-namespace impl {
+namespace number::impl {
 class SimpleModifier;
 }
-}
 
 /**
  * Formats simple patterns like "{1} was born in {0}".
diff --git a/icu4c/source/i18n/double-conversion-cached-powers.cpp b/icu4c/source/i18n/double-conversion-cached-powers.cpp
index 3bc35c8aaf8..ad6ea927309 100644
--- a/icu4c/source/i18n/double-conversion-cached-powers.cpp
+++ b/icu4c/source/i18n/double-conversion-cached-powers.cpp
@@ -47,9 +47,7 @@
 // ICU PATCH: Wrap in ICU namespace
 U_NAMESPACE_BEGIN
 
-namespace double_conversion {
-
-namespace PowersOfTenCache {
+namespace double_conversion::PowersOfTenCache {
 
 struct CachedPower {
   uint64_t significand;
@@ -184,9 +182,7 @@ void GetCachedPowerForDecimalExponent(int requested_exponent,
   DOUBLE_CONVERSION_ASSERT(requested_exponent < *found_exponent + kDecimalExponentDistance);
 }
 
-}  // namespace PowersOfTenCache
-
-}  // namespace double_conversion
+} // namespace double_conversion::PowersOfTenCache
 
 // ICU PATCH: Close ICU namespace
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/double-conversion-cached-powers.h b/icu4c/source/i18n/double-conversion-cached-powers.h
index ade27baef8b..78e70378d9a 100644
--- a/icu4c/source/i18n/double-conversion-cached-powers.h
+++ b/icu4c/source/i18n/double-conversion-cached-powers.h
@@ -44,9 +44,7 @@
 // ICU PATCH: Wrap in ICU namespace
 U_NAMESPACE_BEGIN
 
-namespace double_conversion {
-
-namespace PowersOfTenCache {
+namespace double_conversion::PowersOfTenCache {
 
   // Not all powers of ten are cached. The decimal exponent of two neighboring
   // cached numbers will differ by kDecimalExponentDistance.
@@ -71,9 +69,7 @@ namespace PowersOfTenCache {
                                         DiyFp* power,
                                         int* found_exponent);
 
-}  // namespace PowersOfTenCache
-
-}  // namespace double_conversion
+} // namespace double_conversion::PowersOfTenCache
 
 // ICU PATCH: Close ICU namespace
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/measunit_impl.h b/icu4c/source/i18n/measunit_impl.h
index c60ff2fc33b..5ee5c7bb95d 100644
--- a/icu4c/source/i18n/measunit_impl.h
+++ b/icu4c/source/i18n/measunit_impl.h
@@ -14,11 +14,9 @@
 
 U_NAMESPACE_BEGIN
 
-namespace number {
-namespace impl {
+namespace number::impl {
 class LongNameHandler;
 }
-} // namespace number
 
 static const char16_t kDefaultCurrency[] = u"XXX";
 static const char kDefaultCurrency8[] = "XXX";
diff --git a/icu4c/source/i18n/number_affixutils.h b/icu4c/source/i18n/number_affixutils.h
index 5cfde61ffd0..e59096546e1 100644
--- a/icu4c/source/i18n/number_affixutils.h
+++ b/icu4c/source/i18n/number_affixutils.h
@@ -14,8 +14,8 @@
 #include "formatted_string_builder.h"
 #include "unicode/uniset.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 enum AffixPatternState {
     STATE_BASE = 0,
@@ -234,8 +234,7 @@ class U_I18N_API AffixUtils {
     }
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 
diff --git a/icu4c/source/i18n/number_asformat.h b/icu4c/source/i18n/number_asformat.h
index f921b429420..f447e0c4b0a 100644
--- a/icu4c/source/i18n/number_asformat.h
+++ b/icu4c/source/i18n/number_asformat.h
@@ -18,8 +18,8 @@
 #include "decNumber.h"
 #include "charstr.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 /**
  * A wrapper around LocalizedNumberFormatter implementing the Format interface, enabling improved
@@ -97,8 +97,7 @@ class U_I18N_API LocalizedNumberFormatterAsFormat : public Format {
     Locale fLocale;
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif // __NUMBER_ASFORMAT_H__
diff --git a/icu4c/source/i18n/number_capi.cpp b/icu4c/source/i18n/number_capi.cpp
index abada9ad869..bc7dc8c4342 100644
--- a/icu4c/source/i18n/number_capi.cpp
+++ b/icu4c/source/i18n/number_capi.cpp
@@ -25,8 +25,7 @@ using namespace icu::number::impl;
 
 
 U_NAMESPACE_BEGIN
-namespace number {
-namespace impl {
+namespace number::impl {
 
 /**
  * Implementation class for UNumberFormatter. Wraps a LocalizedNumberFormatter.
@@ -84,8 +83,7 @@ void UFormattedNumberImpl::setTo(FormattedNumber value) {
     fData = std::move(*value.fData);
 }
 
-}
-}
+} // namespace number::impl
 U_NAMESPACE_END
 
 
diff --git a/icu4c/source/i18n/number_compact.h b/icu4c/source/i18n/number_compact.h
index aee1df74523..95896c029a2 100644
--- a/icu4c/source/i18n/number_compact.h
+++ b/icu4c/source/i18n/number_compact.h
@@ -14,8 +14,8 @@
 #include "resource.h"
 #include "number_patternmodifier.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 static const int32_t COMPACT_MAX_DIGITS = 20;
 
@@ -90,9 +90,7 @@ class CompactHandler : public MicroPropsGenerator, public UMemory {
     void precomputeAllModifiers(MutablePatternModifier &buildReference, UErrorCode &status);
 };
 
-
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__NUMBER_COMPACT_H__
diff --git a/icu4c/source/i18n/number_currencysymbols.h b/icu4c/source/i18n/number_currencysymbols.h
index c2223bd0f0b..13b9a9691be 100644
--- a/icu4c/source/i18n/number_currencysymbols.h
+++ b/icu4c/source/i18n/number_currencysymbols.h
@@ -11,9 +11,8 @@
 #include "charstr.h"
 #include "number_decimfmtprops.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
-
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 // Exported as U_I18N_API for tests
 class U_I18N_API CurrencySymbols : public UMemory {
@@ -62,9 +61,7 @@ class U_I18N_API CurrencySymbols : public UMemory {
 CurrencyUnit
 resolveCurrency(const DecimalFormatProperties& properties, const Locale& locale, UErrorCode& status);
 
-
-} // namespace impl
-} // namespace numparse
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__SOURCE_NUMBER_CURRENCYSYMBOLS_H__
diff --git a/icu4c/source/i18n/number_decimalquantity.h b/icu4c/source/i18n/number_decimalquantity.h
index 2211fe9c19e..f3eb17d9225 100644
--- a/icu4c/source/i18n/number_decimalquantity.h
+++ b/icu4c/source/i18n/number_decimalquantity.h
@@ -13,8 +13,8 @@
 #include "plurrule_impl.h"
 #include "number_types.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 // Forward-declare (maybe don't want number_utils.h included here):
 class DecNum;
@@ -549,8 +549,7 @@ class U_I18N_API DecimalQuantity : public IFixedDecimal, public UMemory {
     void switchStorage();
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 
diff --git a/icu4c/source/i18n/number_decimfmtprops.h b/icu4c/source/i18n/number_decimfmtprops.h
index 5f72f649842..6cf4b4c108a 100644
--- a/icu4c/source/i18n/number_decimfmtprops.h
+++ b/icu4c/source/i18n/number_decimfmtprops.h
@@ -33,8 +33,7 @@ template class U_I18N_API LocalPointer<CurrencyPluralInfo>;
 #endif
 #endif
 
-namespace number {
-namespace impl {
+namespace number::impl {
 
 // Exported as U_I18N_API because it is a public member field of exported DecimalFormatProperties
 // Using this wrapper is rather unfortunate, but is needed on Windows platforms in order to allow
@@ -166,8 +165,8 @@ struct U_I18N_API DecimalFormatProperties : public UMemory {
     bool _equals(const DecimalFormatProperties& other, bool ignoreForFastFormat) const;
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
+
 U_NAMESPACE_END
 
 
diff --git a/icu4c/source/i18n/number_decnum.h b/icu4c/source/i18n/number_decnum.h
index 94a0b31bcb5..2bf9fd39cd9 100644
--- a/icu4c/source/i18n/number_decnum.h
+++ b/icu4c/source/i18n/number_decnum.h
@@ -22,8 +22,7 @@ U_NAMESPACE_BEGIN
 template class U_I18N_API MaybeStackHeaderAndArray<decNumber, char, DECNUM_INITIAL_CAPACITY>;
 #endif
 
-namespace number {
-namespace impl {
+namespace number::impl {
 
 /** A very thin C++ wrapper around decNumber.h */
 // Exported as U_I18N_API for tests
@@ -84,8 +83,7 @@ class U_I18N_API DecNum : public UMemory {
     void _setTo(const char* str, int32_t maxDigits, UErrorCode& status);
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 
 U_NAMESPACE_END
 
diff --git a/icu4c/source/i18n/number_formatimpl.h b/icu4c/source/i18n/number_formatimpl.h
index 62d53212616..ba979d8bdc6 100644
--- a/icu4c/source/i18n/number_formatimpl.h
+++ b/icu4c/source/i18n/number_formatimpl.h
@@ -18,8 +18,8 @@
 #include "number_microprops.h"
 #include "number_utypes.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 /**
  * This is the "brain" of the number formatting pipeline. It ties all the pieces together, taking in a MacroProps and a
@@ -170,8 +170,7 @@ class NumberFormatterImpl : public UMemory {
         UErrorCode &status);
 };
 
-}  // namespace impl
-}  // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 
diff --git a/icu4c/source/i18n/number_longnames.h b/icu4c/source/i18n/number_longnames.h
index 56d8c9b24e8..ef1ce74104f 100644
--- a/icu4c/source/i18n/number_longnames.h
+++ b/icu4c/source/i18n/number_longnames.h
@@ -13,8 +13,8 @@
 #include "number_utils.h"
 #include "number_modifiers.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 // LongNameHandler takes care of formatting currency and measurement unit names,
 // as well as populating the gender of measure units.
@@ -263,8 +263,7 @@ class LongNameMultiplexer : public MicroPropsGenerator, public UMemory {
     }
 };
 
-}  // namespace impl
-}  // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__NUMBER_LONGNAMES_H__
diff --git a/icu4c/source/i18n/number_mapper.h b/icu4c/source/i18n/number_mapper.h
index c01607e5688..7c8cf795d5b 100644
--- a/icu4c/source/i18n/number_mapper.h
+++ b/icu4c/source/i18n/number_mapper.h
@@ -16,9 +16,7 @@
 #include "numparse_impl.h"
 
 U_NAMESPACE_BEGIN
-namespace number {
-namespace impl {
-
+namespace number::impl {
 
 class AutoAffixPatternProvider;
 class CurrencyPluralInfoAffixProvider;
@@ -257,9 +255,7 @@ class NumberPropertyMapper {
                                DecimalFormatProperties* exportedProperties, UErrorCode& status);
 };
 
-
-} // namespace impl
-} // namespace numparse
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__NUMBER_MAPPER_H__
diff --git a/icu4c/source/i18n/number_microprops.h b/icu4c/source/i18n/number_microprops.h
index 18addaae087..6d58bc3c360 100644
--- a/icu4c/source/i18n/number_microprops.h
+++ b/icu4c/source/i18n/number_microprops.h
@@ -20,8 +20,8 @@
 #include "charstr.h"
 #include "util.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 /**
  * A copyable container for the integer values of mixed unit measurements.
@@ -188,8 +188,7 @@ struct MicroProps : public MicroPropsGenerator {
     bool exhausted = false;
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif // __NUMBER_MICROPROPS_H__
diff --git a/icu4c/source/i18n/number_modifiers.h b/icu4c/source/i18n/number_modifiers.h
index 9df788c260f..1afee495de1 100644
--- a/icu4c/source/i18n/number_modifiers.h
+++ b/icu4c/source/i18n/number_modifiers.h
@@ -15,8 +15,8 @@
 #include "formatted_string_builder.h"
 #include "number_types.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 /**
  * The canonical implementation of {@link Modifier}, containing a prefix and suffix string.
@@ -350,8 +350,7 @@ class U_I18N_API AdoptingModifierStore : public ModifierStore, public UMemory {
     AdoptingSignumModifierStore mods[StandardPlural::COUNT] = {};
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 
diff --git a/icu4c/source/i18n/number_multiplier.h b/icu4c/source/i18n/number_multiplier.h
index c752935b783..62b04778e85 100644
--- a/icu4c/source/i18n/number_multiplier.h
+++ b/icu4c/source/i18n/number_multiplier.h
@@ -10,9 +10,8 @@
 #include "numparse_types.h"
 #include "number_decimfmtprops.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
-
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 /**
  * Wraps a {@link Multiplier} for use in the number formatting pipeline.
@@ -48,9 +47,7 @@ static inline Scale scaleFromProperties(const DecimalFormatProperties& propertie
     }
 }
 
-
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__SOURCE_NUMBER_MULTIPLIER_H__
diff --git a/icu4c/source/i18n/number_patternmodifier.h b/icu4c/source/i18n/number_patternmodifier.h
index f869ed75d45..4cae160049d 100644
--- a/icu4c/source/i18n/number_patternmodifier.h
+++ b/icu4c/source/i18n/number_patternmodifier.h
@@ -33,8 +33,7 @@ template class U_I18N_API LocalPointer<number::impl::AdoptingModifierStore>;
 #endif
 #endif
 
-namespace number {
-namespace impl {
+namespace number::impl {
 
 // Forward declaration
 class MutablePatternModifier;
@@ -255,9 +254,8 @@ class U_I18N_API MutablePatternModifier
     void prepareAffix(bool isPrefix);
 };
 
+} // namespace number::impl
 
-}  // namespace impl
-}  // namespace number
 U_NAMESPACE_END
 
 #endif //__NUMBER_PATTERNMODIFIER_H__
diff --git a/icu4c/source/i18n/number_patternstring.h b/icu4c/source/i18n/number_patternstring.h
index 989d0d3464b..2b3ab02be0f 100644
--- a/icu4c/source/i18n/number_patternstring.h
+++ b/icu4c/source/i18n/number_patternstring.h
@@ -16,8 +16,8 @@
 #include "number_decimfmtprops.h"
 #include "number_affixutils.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 // Forward declaration
 class PatternParser;
@@ -330,8 +330,7 @@ class U_I18N_API PatternStringUtils {
                                    UErrorCode& status);
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 
diff --git a/icu4c/source/i18n/number_roundingutils.h b/icu4c/source/i18n/number_roundingutils.h
index 66571272545..08c62dd6aee 100644
--- a/icu4c/source/i18n/number_roundingutils.h
+++ b/icu4c/source/i18n/number_roundingutils.h
@@ -11,8 +11,7 @@
 #include "string_segment.h"
 
 U_NAMESPACE_BEGIN
-namespace number {
-namespace impl {
+namespace number::impl {
 namespace roundingutils {
 
 enum Section {
@@ -238,8 +237,7 @@ class RoundingImpl {
  */
 void parseIncrementOption(const StringSegment &segment, Precision &outPrecision, UErrorCode &status);
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__NUMBER_ROUNDINGUTILS_H__
diff --git a/icu4c/source/i18n/number_scientific.h b/icu4c/source/i18n/number_scientific.h
index 4dbc031102f..60122b15511 100644
--- a/icu4c/source/i18n/number_scientific.h
+++ b/icu4c/source/i18n/number_scientific.h
@@ -9,8 +9,8 @@
 
 #include "number_types.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 // Forward-declare
 class ScientificHandler;
@@ -59,8 +59,7 @@ class ScientificHandler : public UMemory, public MicroPropsGenerator, public Mul
     friend class ScientificModifier;
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__NUMBER_SCIENTIFIC_H__
diff --git a/icu4c/source/i18n/number_skeletons.h b/icu4c/source/i18n/number_skeletons.h
index 27f69cd48c3..47a1a7ace89 100644
--- a/icu4c/source/i18n/number_skeletons.h
+++ b/icu4c/source/i18n/number_skeletons.h
@@ -13,8 +13,7 @@
 #include "string_segment.h"
 
 U_NAMESPACE_BEGIN
-namespace number {
-namespace impl {
+namespace number::impl {
 
 // Forward-declaration
 struct SeenMacroProps;
@@ -385,8 +384,7 @@ UPRV_BLOCK_MACRO_BEGIN { \
 
 } // namespace
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__SOURCE_NUMBER_SKELETONS_H__
diff --git a/icu4c/source/i18n/number_types.h b/icu4c/source/i18n/number_types.h
index 825a3c455c9..a67099e5e3b 100644
--- a/icu4c/source/i18n/number_types.h
+++ b/icu4c/source/i18n/number_types.h
@@ -20,8 +20,7 @@
 #include "formatted_string_builder.h"
 
 U_NAMESPACE_BEGIN
-namespace number {
-namespace impl {
+namespace number::impl {
 
 // For convenience and historical reasons, import the Field typedef to the namespace.
 typedef FormattedStringBuilder::Field Field;
@@ -369,9 +368,7 @@ class U_I18N_API NullableValue {
     T fValue;
 };
 
-
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__NUMBER_TYPES_H__
diff --git a/icu4c/source/i18n/number_usageprefs.h b/icu4c/source/i18n/number_usageprefs.h
index e90df99d396..9aebc3140cd 100644
--- a/icu4c/source/i18n/number_usageprefs.h
+++ b/icu4c/source/i18n/number_usageprefs.h
@@ -23,8 +23,7 @@ U_NAMESPACE_BEGIN
 using ::icu::units::ComplexUnitsConverter;
 using ::icu::units::UnitsRouter;
 
-namespace number {
-namespace impl {
+namespace number::impl {
 
 /**
  * A MicroPropsGenerator which uses UnitsRouter to produce output converted to a
@@ -62,8 +61,7 @@ class U_I18N_API UsagePrefsHandler : public MicroPropsGenerator, public UMemory
     const MicroPropsGenerator *fParent;
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 
 // Export explicit template instantiations of LocalPointerBase and LocalPointer.
 // This is required when building DLLs for Windows. (See datefmt.h,
@@ -84,8 +82,7 @@ template class U_I18N_API LocalPointer<ComplexUnitsConverter>;
 #endif
 #endif
 
-namespace number {
-namespace impl {
+namespace number::impl {
 
 /**
  * A MicroPropsGenerator which converts a measurement from one MeasureUnit to
@@ -118,8 +115,8 @@ class U_I18N_API UnitConversionHandler : public MicroPropsGenerator, public UMem
     const MicroPropsGenerator *fParent;
 };
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
+
 U_NAMESPACE_END
 
 #endif // __NUMBER_USAGEPREFS_H__
diff --git a/icu4c/source/i18n/number_utils.h b/icu4c/source/i18n/number_utils.h
index bc369c940f7..39c56f4d184 100644
--- a/icu4c/source/i18n/number_utils.h
+++ b/icu4c/source/i18n/number_utils.h
@@ -21,8 +21,7 @@
 
 U_NAMESPACE_BEGIN
 
-namespace number {
-namespace impl {
+namespace number::impl {
 
 enum CldrPatternStyle {
     CLDR_PATTERN_STYLE_DECIMAL,
@@ -102,8 +101,7 @@ inline StandardPlural::Form getPluralSafe(
 
 } // namespace utils
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 
 U_NAMESPACE_END
 
diff --git a/icu4c/source/i18n/number_utypes.h b/icu4c/source/i18n/number_utypes.h
index 0c130401894..9f606f17891 100644
--- a/icu4c/source/i18n/number_utypes.h
+++ b/icu4c/source/i18n/number_utypes.h
@@ -13,9 +13,8 @@
 #include "formatted_string_builder.h"
 #include "formattedval_impl.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
-
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 /** Helper function used in upluralrules.cpp */
 const DecimalQuantity* validateUFormattedNumberToDecimalQuantity(
@@ -50,9 +49,7 @@ public:
     const char *gender = "";
 };
 
-
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__SOURCE_NUMBER_UTYPES_H__
diff --git a/icu4c/source/i18n/numparse_affixes.h b/icu4c/source/i18n/numparse_affixes.h
index 81b633c2626..a9e1c414e8c 100644
--- a/icu4c/source/i18n/numparse_affixes.h
+++ b/icu4c/source/i18n/numparse_affixes.h
@@ -16,8 +16,8 @@
 #include "number_currencysymbols.h"
 
 U_NAMESPACE_BEGIN
-namespace numparse {
-namespace impl {
+
+namespace numparse::impl {
 
 // Forward-declaration of implementation classes for friending
 class AffixPatternMatcherBuilder;
@@ -44,8 +44,7 @@ class U_I18N_API CodePointMatcher : public NumberParseMatcher, public UMemory {
     UChar32 fCp;
 };
 
-} // namespace impl
-} // namespace numparse
+} // namespace numparse::impl
 
 // Export a explicit template instantiations of MaybeStackArray, MemoryPool and CompactUnicodeString.
 // When building DLLs for Windows this is required even though no direct access leaks out of the i18n library.
@@ -58,8 +57,7 @@ template class U_I18N_API MemoryPool<numparse::impl::CodePointMatcher, 8>;
 template class U_I18N_API numparse::impl::CompactUnicodeString<4>;
 #endif
 
-namespace numparse {
-namespace impl {
+namespace numparse::impl {
 
 struct AffixTokenMatcherSetupData {
     const CurrencySymbols& currencySymbols;
@@ -221,9 +219,8 @@ class AffixMatcherWarehouse {
                               parse_flags_t parseFlags, UErrorCode& status);
 };
 
+} // namespace numparse::impl
 
-} // namespace impl
-} // namespace numparse
 U_NAMESPACE_END
 
 #endif //__NUMPARSE_AFFIXES_H__
diff --git a/icu4c/source/i18n/numparse_compositions.h b/icu4c/source/i18n/numparse_compositions.h
index f085912def1..b21941deec3 100644
--- a/icu4c/source/i18n/numparse_compositions.h
+++ b/icu4c/source/i18n/numparse_compositions.h
@@ -18,8 +18,7 @@ U_NAMESPACE_BEGIN
 template class U_I18N_API MaybeStackArray<const numparse::impl::NumberParseMatcher*, 3>;
 #endif
 
-namespace numparse {
-namespace impl {
+namespace numparse::impl {
 
 /**
  * Base class for AnyMatcher and SeriesMatcher.
@@ -115,9 +114,8 @@ class U_I18N_API ArraySeriesMatcher : public SeriesMatcher {
     int32_t fMatchersLen;
 };
 
+} // namespace numparse::impl
 
-} // namespace impl
-} // namespace numparse
 U_NAMESPACE_END
 
 #endif //__SOURCE_NUMPARSE_COMPOSITIONS__
diff --git a/icu4c/source/i18n/numparse_currency.h b/icu4c/source/i18n/numparse_currency.h
index 4e99334a31b..4d71d4fbc91 100644
--- a/icu4c/source/i18n/numparse_currency.h
+++ b/icu4c/source/i18n/numparse_currency.h
@@ -13,8 +13,8 @@
 #include "number_currencysymbols.h"
 #include "unicode/uniset.h"
 
-U_NAMESPACE_BEGIN namespace numparse {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace numparse::impl {
 
 using ::icu::number::impl::CurrencySymbols;
 
@@ -65,9 +65,7 @@ class U_I18N_API CombinedCurrencyMatcher : public NumberParseMatcher, public UMe
     bool matchCurrency(StringSegment& segment, ParsedNumber& result, UErrorCode& status) const;
 };
 
-
-} // namespace impl
-} // namespace numparse
+} // namespace numparse::impl
 U_NAMESPACE_END
 
 #endif //__NUMPARSE_CURRENCY_H__
diff --git a/icu4c/source/i18n/numparse_decimal.h b/icu4c/source/i18n/numparse_decimal.h
index 07c9afeccc9..b770749fa01 100644
--- a/icu4c/source/i18n/numparse_decimal.h
+++ b/icu4c/source/i18n/numparse_decimal.h
@@ -10,8 +10,8 @@
 #include "unicode/uniset.h"
 #include "numparse_types.h"
 
-U_NAMESPACE_BEGIN namespace numparse {
-namespace impl {
+U_NAMESPACE_BEGIN
+namespace numparse::impl {
 
 using ::icu::number::impl::Grouper;
 
@@ -67,9 +67,7 @@ class DecimalMatcher : public NumberParseMatcher, public UMemory {
     bool validateGroup(int32_t sepType, int32_t count, bool isPrimary) const;
 };
 
-
-} // namespace impl
-} // namespace numparse
+} // namespace numparse::impl
 U_NAMESPACE_END
 
 #endif //__NUMPARSE_DECIMAL_H__
diff --git a/icu4c/source/i18n/numparse_impl.h b/icu4c/source/i18n/numparse_impl.h
index 380d9aa4c64..4695a624d65 100644
--- a/icu4c/source/i18n/numparse_impl.h
+++ b/icu4c/source/i18n/numparse_impl.h
@@ -29,8 +29,7 @@ U_NAMESPACE_BEGIN
 template class U_I18N_API MaybeStackArray<const numparse::impl::NumberParseMatcher*, 10>;
 #endif
 
-namespace numparse {
-namespace impl {
+namespace numparse::impl {
 
 // Exported as U_I18N_API for tests
 class U_I18N_API NumberParserImpl : public MutableMatcherCollection, public UMemory {
@@ -102,9 +101,8 @@ class U_I18N_API NumberParserImpl : public MutableMatcherCollection, public UMem
         StringSegment& segment, ParsedNumber& result, int32_t recursionLevels, UErrorCode& status) const;
 };
 
+} // namespace numparse::impl
 
-} // namespace impl
-} // namespace numparse
 U_NAMESPACE_END
 
 #endif //__NUMPARSE_IMPL_H__
diff --git a/icu4c/source/i18n/numparse_scientific.h b/icu4c/source/i18n/numparse_scientific.h
index 5617c0c6a60..ed214b71711 100644
--- a/icu4c/source/i18n/numparse_scientific.h
+++ b/icu4c/source/i18n/numparse_scientific.h
@@ -14,9 +14,8 @@
 
 using icu::number::impl::Grouper;
 
-U_NAMESPACE_BEGIN namespace numparse {
-namespace impl {
-
+U_NAMESPACE_BEGIN
+namespace numparse::impl {
 
 class ScientificMatcher : public NumberParseMatcher, public UMemory {
   public:
@@ -38,9 +37,7 @@ class ScientificMatcher : public NumberParseMatcher, public UMemory {
     UnicodeString fCustomPlusSign;
 };
 
-
-} // namespace impl
-} // namespace numparse
+} // namespace numparse::impl
 U_NAMESPACE_END
 
 #endif //__NUMPARSE_SCIENTIFIC_H__
diff --git a/icu4c/source/i18n/numparse_symbols.h b/icu4c/source/i18n/numparse_symbols.h
index beb133f7d07..3bc07b2be39 100644
--- a/icu4c/source/i18n/numparse_symbols.h
+++ b/icu4c/source/i18n/numparse_symbols.h
@@ -11,9 +11,8 @@
 #include "unicode/uniset.h"
 #include "static_unicode_sets.h"
 
-U_NAMESPACE_BEGIN namespace numparse {
-namespace impl {
-
+U_NAMESPACE_BEGIN
+namespace numparse::impl {
 
 /**
  * A base class for many matchers that performs a simple match against a UnicodeString and/or UnicodeSet.
@@ -164,9 +163,7 @@ class U_I18N_API PlusSignMatcher : public SymbolMatcher {
     bool fAllowTrailing;
 };
 
-
-} // namespace impl
-} // namespace numparse
+} // namespace numparse::impl
 U_NAMESPACE_END
 
 #endif //__NUMPARSE_SYMBOLS_H__
diff --git a/icu4c/source/i18n/numparse_types.h b/icu4c/source/i18n/numparse_types.h
index 8e881793fd3..d5a53379be1 100644
--- a/icu4c/source/i18n/numparse_types.h
+++ b/icu4c/source/i18n/numparse_types.h
@@ -12,8 +12,7 @@
 #include "string_segment.h"
 
 U_NAMESPACE_BEGIN
-namespace numparse {
-namespace impl {
+namespace numparse::impl {
 
 // Forward-declarations
 class ParsedNumber;
@@ -263,9 +262,7 @@ class U_I18N_API MutableMatcherCollection {
     virtual void addMatcher(NumberParseMatcher& matcher) = 0;
 };
 
-
-} // namespace impl
-} // namespace numparse
+} // namespace numparse::impl
 U_NAMESPACE_END
 
 #endif //__NUMPARSE_TYPES_H__
diff --git a/icu4c/source/i18n/numparse_utils.h b/icu4c/source/i18n/numparse_utils.h
index 8fda4f4369b..a842c6fae0c 100644
--- a/icu4c/source/i18n/numparse_utils.h
+++ b/icu4c/source/i18n/numparse_utils.h
@@ -10,10 +10,8 @@
 #include "numparse_types.h"
 #include "unicode/uniset.h"
 
-U_NAMESPACE_BEGIN namespace numparse {
-namespace impl {
-namespace utils {
-
+U_NAMESPACE_BEGIN
+namespace numparse::impl::utils {
 
 inline static void putLeadCodePoints(const UnicodeSet* input, UnicodeSet* output) {
     for (int32_t i = 0; i < input->getRangeCount(); i++) {
@@ -33,10 +31,7 @@ inline static void copyCurrencyCode(char16_t* dest, const char16_t* src) {
     dest[3] = 0;
 }
 
-
-} // namespace utils
-} // namespace impl
-} // namespace numparse
+} // namespace numparse::impl::utils
 U_NAMESPACE_END
 
 #endif //__NUMPARSE_UTILS_H__
diff --git a/icu4c/source/i18n/numparse_validators.h b/icu4c/source/i18n/numparse_validators.h
index 9bb4b482b55..3425bb07a79 100644
--- a/icu4c/source/i18n/numparse_validators.h
+++ b/icu4c/source/i18n/numparse_validators.h
@@ -10,9 +10,8 @@
 #include "numparse_types.h"
 #include "static_unicode_sets.h"
 
-U_NAMESPACE_BEGIN namespace numparse {
-namespace impl {
-
+U_NAMESPACE_BEGIN
+namespace numparse::impl {
 
 class ValidationMatcher : public NumberParseMatcher {
   public:
@@ -86,9 +85,7 @@ class MultiplierParseHandler : public ValidationMatcher, public UMemory {
     ::icu::number::Scale fMultiplier;
 };
 
-
-} // namespace impl
-} // namespace numparse
+} // namespace numparse::impl
 U_NAMESPACE_END
 
 #endif //__SOURCE_NUMPARSE_VALIDATORS_H__
diff --git a/icu4c/source/i18n/numrange_capi.cpp b/icu4c/source/i18n/numrange_capi.cpp
index 9222969eb49..1432ea89ccd 100644
--- a/icu4c/source/i18n/numrange_capi.cpp
+++ b/icu4c/source/i18n/numrange_capi.cpp
@@ -24,8 +24,7 @@ using namespace icu::number::impl;
 
 
 U_NAMESPACE_BEGIN
-namespace number {
-namespace impl {
+namespace number::impl {
 
 /**
  * Implementation class for UNumberRangeFormatter. Wraps a LocalizedRangeNumberFormatter.
@@ -59,8 +58,7 @@ UFormattedNumberRangeImpl::~UFormattedNumberRangeImpl() {
     fImpl.fData = nullptr;
 }
 
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 
diff --git a/icu4c/source/i18n/numrange_impl.h b/icu4c/source/i18n/numrange_impl.h
index ac1d8a58972..cdee3d84ee7 100644
--- a/icu4c/source/i18n/numrange_impl.h
+++ b/icu4c/source/i18n/numrange_impl.h
@@ -17,9 +17,8 @@
 #include "formattedval_impl.h"
 #include "pluralranges.h"
 
-U_NAMESPACE_BEGIN namespace number {
-namespace impl {
-
+U_NAMESPACE_BEGIN
+namespace number::impl {
 
 /**
  * Class similar to UFormattedNumberData.
@@ -80,9 +79,7 @@ class NumberRangeFormatterImpl : public UMemory {
 const UFormattedNumberRangeData* validateUFormattedNumberRange(
     const UFormattedNumberRange* uresult, UErrorCode& status);
 
-
-} // namespace impl
-} // namespace number
+} // namespace number::impl
 U_NAMESPACE_END
 
 #endif //__SOURCE_NUMRANGE_TYPES_H__
diff --git a/icu4c/source/i18n/pluralranges.h b/icu4c/source/i18n/pluralranges.h
index eba59c70ed7..c06317850ae 100644
--- a/icu4c/source/i18n/pluralranges.h
+++ b/icu4c/source/i18n/pluralranges.h
@@ -17,11 +17,9 @@
 U_NAMESPACE_BEGIN
 
 // Forward declarations
-namespace number {
-namespace impl {
+namespace number::impl {
 class UFormattedNumberRangeData;
 }
-}
 
 class StandardPluralRanges : public UMemory {
   public:
diff --git a/icu4c/source/i18n/unicode/decimfmt.h b/icu4c/source/i18n/unicode/decimfmt.h
index 713e4680959..1c55e367780 100644
--- a/icu4c/source/i18n/unicode/decimfmt.h
+++ b/icu4c/source/i18n/unicode/decimfmt.h
@@ -60,11 +60,9 @@ class UFormattedNumberData;
 }
 }
 
-namespace numparse {
-namespace impl {
+namespace numparse::impl {
 class NumberParserImpl;
 }
-}
 
 /**
  * **IMPORTANT:** New users are strongly encouraged to see if
diff --git a/icu4c/source/i18n/unicode/fmtable.h b/icu4c/source/i18n/unicode/fmtable.h
index 2efb1f758ab..d251d35ba88 100644
--- a/icu4c/source/i18n/unicode/fmtable.h
+++ b/icu4c/source/i18n/unicode/fmtable.h
@@ -35,11 +35,10 @@
 U_NAMESPACE_BEGIN
 
 class CharString;
-namespace number {
-namespace impl {
+
+namespace number::impl {
 class DecimalQuantity;
 }
-}
 
 /**
  * Formattable objects can be passed to the Format class or
diff --git a/icu4c/source/i18n/unicode/measunit.h b/icu4c/source/i18n/unicode/measunit.h
index 5dba69e6557..9ce9b1a65d0 100644
--- a/icu4c/source/i18n/unicode/measunit.h
+++ b/icu4c/source/i18n/unicode/measunit.h
@@ -33,11 +33,9 @@ U_NAMESPACE_BEGIN
 class StringEnumeration;
 class MeasureUnitImpl;
 
-namespace number {
-namespace impl {
+namespace number::impl {
 class LongNameHandler;
-}
-} // namespace number
+} // namespace number::impl
 
 /**
  * Enumeration for unit complexity. There are three levels:
diff --git a/icu4c/source/i18n/unicode/numberformatter.h b/icu4c/source/i18n/unicode/numberformatter.h
index b0566c2d92b..b02d987ce2b 100644
--- a/icu4c/source/i18n/unicode/numberformatter.h
+++ b/icu4c/source/i18n/unicode/numberformatter.h
@@ -93,15 +93,13 @@ class IFixedDecimal;
 class FieldPositionIteratorHandler;
 class FormattedStringBuilder;
 
-namespace numparse {
-namespace impl {
+namespace numparse::impl {
 
 // Forward declarations:
 class NumberParserImpl;
 class MultiplierParseHandler;
 
-}
-}
+} // namespace numparse::impl
 
 namespace units {