-.Sh COMPATIBILITY
-This section documents areas of questionable portability between
-implementations of the
-.Nm
-language.
-.Pp
-.Bl -dash -compact
-.It
-Do not depend on
-.Sx \&SH
-or
-.Sx \&SS
-to close out a literal context opened with
-.Sx \&nf .
-This behaviour may not be portable.
-.It
-In quoted literals, GNU troff allowed pair-wise double-quotes to produce
-a standalone double-quote in formatted output.
-It is not known whether this behaviour is exhibited by other formatters.
-.It
-troff suppresses a newline before
-.Sq \(aq
-macro output; in mandoc, it is an alias for the standard
-.Sq \&.
-control character.
-.It
-The
-.Sq \eh
-.Pq horizontal position ,
-.Sq \ev
-.Pq vertical position ,
-.Sq \em
-.Pq text colour ,
-.Sq \eM
-.Pq text filling colour ,
-.Sq \ez
-.Pq zero-length character ,
-.Sq \ew
-.Pq string length ,
-.Sq \ek
-.Pq horizontal position marker ,
-.Sq \eo
-.Pq text overstrike ,
-and
-.Sq \es
-.Pq text size
-escape sequences are all discarded in mandoc.
-.It
-The
-.Sq \ef
-scaling unit is accepted by mandoc, but rendered as the default unit.
-.It
-The
-.Sx \&sp
-macro does not accept negative values in mandoc.
-In GNU troff, this would result in strange behaviour.
-.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 .
-.El
-.Pp
-The
-.Sx OP
-macro is part of the extended
-.Nm
-macro set, and may not be portable to non-GNU troff implementations.