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TeX Frequently Asked Questions
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In the deeps of time, when TeX first happened, computers had extremely limited memory, and were (by today’s standards) painfully slow. When LaTeX came along, things weren’t much better, and even when LaTeX2e appeared, there was a strong imperative to save memory space (and to a lesser extent) CPU time.

From the very earliest days, Knuth used shortcut macros to speed things up. LaTeX, over the years, has extended Knuth’s list by a substantial amount. An interesting feature of the “abbreviations” is that on paper, they may look longer than the thing they stand for; however, to (La)TeX they feel smaller…

The table at the end of this answer lists the commonest of these “abbreviations”. It is not complete; as always, if the table doesn’t help, try the LaTeX source. The table lists each abbreviation’s name and its value, which provide most of what a user needs to know. The table also lists the abbreviation’s type, which is a trickier concept: if you need to know, the only real confusion is that the abbreviations labelled ‘defn’ are defined using an \xxxxdef command.

Name	Type	Value
\m@ne	count	-1
\p@	dimen	1pt
\z@	dimen	0pt
\z@skip	skip	0pt plus 0pt minus 0pt
\@ne	defn	1
\tw@	defn	2
\thr@@	defn	3
\sixt@@n	defn	16
\@cclv	defn	255
\@cclvi	defn	256
\@m	defn	1000
\@M	defn	10000
\@MM	defn	20000
\@vpt	macro	5
\@vipt	macro	6
\@viipt	macro	7
\@viiipt	macro	8
\@ixpt	macro	9
\@xpt	macro	10
\@xipt	macro	10.95
\@xiipt	macro	12
\@xivpt	macro	14.4
\@xviipt	macro	17.28
\@xxpt	macro	20.74
\@xxvpt	macro	24.88
\@plus	macro	“plus”
\@minus	macro	“minus”