As in troff(1), a macro is called by placing a . (dot character) at the beginning of a line followed by the two character name for the macro. Arguments may follow the macro separated by spaces. It is the dot character at the beginning of the line which causes troff(1) to interpret the next two characters as a macro name. To place a . (dot character) at the beginning of a line in some context other than a macro invocation, precede the . (dot) with the \& escape sequence. The \& translates literally to a zero width space, and is never displayed in the output.
In general, troff(1) macros accept up to nine arguments, any extra arguments are ignored. Most macros in -mdoc accept nine arguments and, in limited cases, arguments may be continued or extended on the next line (See Extensions). A few macros handle quoted arguments (see Passing Space Characters in an Argument below).
Most of the -mdoc general text domain and manual domain macros are special in that their argument lists are parsed for callable macro names. This means an argument on the argument list which matches a general text or manual domain macro name and is determined to be callable will be executed or called when it is processed. In this case the argument, although the name of a macro, is not preceded by a . (dot). It is in this manner that many macros are nested; for example the option macro, .Op, may call the flag and argument macros, Fl and Ar, to specify an optional flag with an argument: