Horizontal Spacing

In horizontal spacing, an em space measures the width of printed matter. All the other spaces are based on the em space.

em space equal to typesize, i.e. an em space in 12-point text is 12 points wide 2003
en space half of an em space 2002
thick space a third of an em space 2004
medium space/mid space/word space a fourth of an em space 2005
thin space a fifth or a sixth of an em space 2009
hair space thinner than a thin space (sometimes by half) 200A

To produce the space, type the code on the numeric keypad, then press Alt together with x.