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There are two types of predefined registers: read-only and read-write. These are all accessed via the \n escape sequence, even though some of them actually return string values.
| .$ | 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. |
| % | Current page number. |
| ct | Character type (set by \w function). |
| dl | Width (maximum) of last completed diversion. |
| dn | Height (vertical size) of last completed diversion. |
| dw | Current day of the week (1 to 7). |
| dy | Current day of the month (1 to 31). |
| hp | Current horizontal place on input line. |
| ln | Output line number. |
| mo | Current month (1 to 12). |
| nl | Vertical position of last printed text baseline. |
| sb | Depth of string below baseline (generated by \w function). |
| st | Height of string above baseline (generated by \w function). |
| yr | Years since 1900.[2] Yes, there's a potential Y2K problem here. |
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| 12.7. Escape Sequences |  | 12.9. Special Characters |
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