Predefined Registers (UNIX in a Nutshell: System V Edition)

`.$`	Number of arguments available at the current macro level.
`$$`	Process ID of `troff` process (`ditroff` only).
`.A`	Set to 1 in `troff`, if `-a` option used; always 1 in `nroff`.
`.F`	Name of the current input file (recent `ditroff` only).
`.H`	Available horizontal resolution in basic units.
`.L`	Current line spacing (set by `.ls`) value (recent `ditroff` only).
`.R`	Number of unused number registers (recent `ditroff` only).
`.T`	Set to 1 in `nroff`, if `-T` option used; always 0 in `oroff`; in `ditroff`, the string `\*(.T` contains the value of `-T`.
`.V`	Available vertical resolution in basic units.
`.a`	Post-line extra line space most recently utilized using `\x'``n``'`.
`.b`	Emboldening level (recent `ditroff` only).
`.c`	Number of lines read from current input file.
`.d`	Current vertical place in current diversion; equal to register `nl` when there is no diversion.
`.f`	Current font as number (1 to 4 in `otroff`; 1 to 99 in `ditroff`).
`.h`	Text baseline high-water mark on current page or diversion.
`.i`	Current indent.
`.j`	Current adjustment mode.
`.k`	Current output horizontal position.
`.l`	Current line length.
`.n`	Length of text portion on previous output line.
`.o`	Current page offset.
`.p`	Current page length.
`.s`	Current point size.
`.t`	Distance to the next trap.
`.u`	Equal to 1 in fill mode and 0 in no-fill mode.
`.v`	Current vertical line spacing.
`.w`	Width of previous character.
`.x`	Reserved version-dependent register.
`.y`	Reserved version-dependent register.
`.z`	Name of current diversion.

12.8. Predefined Registers