-You must also define \fB_XOPEN_SOURCE_EXTENDED\fP when compiling for the
-wide-character library to use the extended (wide-character) functions.
+You must also enable the wide-character features in the header file
+when compiling for the wide-character library
+to use the extended (wide-character) functions.
+The symbol which enables these features has changed since XSI Curses, Issue 4:
+.RS
+.bP
+Originally, the wide-character feature required the symbol
+\fB_XOPEN_SOURCE_EXTENDED\fP
+but that was only valid for XPG4 (1996).
+.bP
+Later, that was deemed conflicting with \fB_XOPEN_SOURCE\fP defined to 500.
+.bP
+As of mid-2018,
+none of the features in this implementation require a \fB_XOPEN_SOURCE\fP
+feature greater than 600.
+However, X/Open Curses, Issue 7 (2009) recommends defining it to 700.
+.bP
+Alternatively, you can enable the feature by defining \fBNCURSES_WIDECHAR\fP
+with the caveat that some other header file than \fBcurses.h\fP
+may require a specific value for \fB_XOPEN_SOURCE\fP
+(or a system-specific symbol).
+.RE
+.IP