Use the string 'mC', defined in groff's an-ext.tmac file
.\" register .g is defined to be 1 for heirloom-doc (and groff) .if t \{\ . ie \n(.g \{\ . ds mC CR .\" tm constant width font used is \*(mC .\" String mC is \*(mC . \} . el \{\ . if !'\*(mC'' \{\ . tm String "mC" already defined to be \*(mC . tm Is redefined to be CW . \} . ds mC CW .\". tm constant width font used is \*(mC . \} .\} then use "\ef(\*(mC" to change the font, for example \f(\*(mCString in constant width\fP Such a definition is then added after the '.TH' macro call. Testet with groff and heirloom-doc only.