Em

A relative unit equal to the current type size, used for spacing, indents and dashes.

Every other unit in this archive has a size. The em does not. An em is equal to whatever the type size happens to be — in twelve point type an em is twelve points wide, in thirty-six point type it is thirty-six. It is a ratio wearing the clothes of a measurement.

That is exactly what a compositor needed. Indents, spaces and dashes have to scale with the type or the page falls apart, so a unit that resizes itself is more useful than one that does not. The name is usually explained as the width of a capital M, which is roughly right in many faces and is the standard account rather than a documented fact.

Half an em is an en. Both survive in punctuation that most people use without knowing why: the en dash and the em dash are precisely those widths, which is the only reason they have different names.

And the em is still working. Web pages are sized in CSS units called em and rem, meaning what they meant to a man setting metal by hand — a measure relative to the size of the type itself. It may be the oldest unit in this archive still doing its original job, on the page you are reading it on.