diff --git a/NEWS b/NEWS
index 520db40..9e30482 100644
--- a/NEWS
+++ b/NEWS
@@ -35,6 +35,30 @@ Major new features:
   are the same interfaces added in version 2.26 for some platforms where
   this format is supported but is not the format of long double.
 
+* Support for two grammatical forms of month name has been added.
+  In a call to strftime, the "%B" and "%b" format specifiers will now
+  produce the grammatical form required when the month is used as part
+  of a complete date.  New "%OB" and "%Ob" specifiers produce the form
+  required when the month is named by itself.  For instance, in Greek
+  and in many Slavic and Baltic languages, "%B" will produce the month
+  in genitive case, and "%OB" will produce the month in nominative case.
+
+  In a call to strptime, "%B", "%b", "%h", "%OB", "%Ob", and "%Oh"
+  are all valid and will all accept any known form of month
+  name---standalone or complete, abbreviated or full.  In a call to
+  nl_langinfo, the query constants MON_1..12 and ABMON_1..12 return
+  the strings used by "%B" and "%b", respectively.  New query
+  constants ALTMON_1..12 and _NL_ABALTMON_1..12 return the strings
+  used by "%OB" and "%Ob", respectively.
+
+  In a locale definition file, use "alt_mon" and "ab_alt_mon" to
+  define the strings for %OB and %Ob, respectively; these have the
+  same syntax as "mon" and "ab_mon".
+
+  This feature is currently a GNU extension, but it is expected to
+  be added to the next revision of POSIX, and it is also already
+  available on some BSD-derived operating systems.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * On GNU/Linux, the obsolete Linux constant PTRACE_SEIZE_DEVEL is no longer
diff --git a/manual/locale.texi b/manual/locale.texi
index f7a40c2..e5cd9d7 100644
--- a/manual/locale.texi
+++ b/manual/locale.texi
@@ -923,7 +923,7 @@ corresponds to Sunday.
 @itemx DAY_5
 @itemx DAY_6
 @itemx DAY_7
-Similar to @code{ABDAY_1} etc., but here the return value is the
+Similar to @code{ABDAY_1} etc.,@: but here the return value is the
 unabbreviated weekday name.
 @item ABMON_1
 @itemx ABMON_2
@@ -937,7 +937,8 @@ unabbreviated weekday name.
 @itemx ABMON_10
 @itemx ABMON_11
 @itemx ABMON_12
-The return value is abbreviated name of the month.  @code{ABMON_1}
+The return value is the abbreviated name of the month, in the grammatical
+form used when the month forms part of a complete date.  @code{ABMON_1}
 corresponds to January.
 @item MON_1
 @itemx MON_2
@@ -951,8 +952,27 @@ corresponds to January.
 @itemx MON_10
 @itemx MON_11
 @itemx MON_12
-Similar to @code{ABMON_1} etc., but here the month names are not abbreviated.
+Similar to @code{ABMON_1} etc.,@: but here the month names are not abbreviated.
 Here the first value @code{MON_1} also corresponds to January.
+@item ALTMON_1
+@itemx ALTMON_2
+@itemx ALTMON_3
+@itemx ALTMON_4
+@itemx ALTMON_5
+@itemx ALTMON_6
+@itemx ALTMON_7
+@itemx ALTMON_8
+@itemx ALTMON_9
+@itemx ALTMON_10
+@itemx ALTMON_11
+@itemx ALTMON_12
+Similar to @code{MON_1} etc.,@: but here the month names are in the grammatical
+form used when the month is named by itself.  The @code{strftime} functions
+use these month names for the format specifier @code{OB}.
+
+Note that not all languages need two different forms of the month names,
+so the strings returned for @code{MON_@dots{}} and @code{ALTMON_@dots{}}
+may or may not be the same, depending on the locale.
 @item AM_STR
 @itemx PM_STR
 The return values are strings which can be used in the representation of time
diff --git a/manual/time.texi b/manual/time.texi
index 33aa221..81c7674 100644
--- a/manual/time.texi
+++ b/manual/time.texi
@@ -1346,8 +1346,13 @@ example, @code{%Ex} might yield a date format based on
the Japanese
 Emperors' reigns.
 
 @item O
-Use the locale's alternate numeric symbols for numbers.  This modifier
-applies only to numeric format specifiers.
+With all format specifiers that produce numbers: use the locale's
+alternate numeric symbols.
+
+With @code{%B} and @code{%b}: use the grammatical form for month names
+that is appropriate when the month is named by itself, rather than
+the form that is appropriate when the month is used as part of a
+complete date.  This is a GNU extension.
 @end table
 
