+.It
+In page header lines, GNU troff versions up to and including 1.21
+only print
+.Ar volume
+names explicitly specified in the
+.Sx \&TH
+macro; mandoc and newer groff print the default volume name
+corresponding to the
+.Ar section
+number when no
+.Ar volume
+is given, like in
+.Xr mdoc 7 .