Math Spacing and Length Units

整理了一下。忘记了也有地方可以查

Math spacing:

\quad	width of M
\!	-3/18 quad
\,	3/18 quad
\:	4/18 quad
\;	5/18 quad
\	6/18 quad
\qquad	2 quad

Lengths defined relative to characters:

em	width of M
ex	width of x
mu		1 mu = 1/18 em

Physical lengths (commonly used):

mm	millimeter
cm	centimeter
in	inch	1 in = 25.4 mm
pt	point	1 pt = 1/72.27 in

Physical lengths (not so commonly used):

bp	big point	1 bp = 1/72 in
pc	pica	1 pc = 12 pt
sp	scaled point	1 sp = 1/65536 pt
dd	didot point	1 dd = 1238/1157 pt
cc	cicero	1 cc = 12 dd