Origins of em?

[vp8317]

First of all, sorry for my bad English; I’m not a native speaker.

I look for historical information about origins of the term em (typographic unit of measurement).

Many sources state that it descends from the letter M, since sometime and somewhere it had exactly square dimensions. This sounds credible, but I fail to find any reliable historical evidences.

For instance, when and where was the term em used for the first time? Who has invented it?

And more important, was this word introduced as a precise metric description (“I mean exactly the size of the letter M”), or an associative one (“I say em because it associates with a letter M which dimensions roughly resemble a square”)? I incline towards the second choice, but I again lack any evidences.

Thanks to all who can point me to respective sources of information.

[Cloud]

definition from wiki

In metal type, the point size (and hence the em) is measured as the height of the metal body from which the letter rises. In metal type (possible overhangs aside), the physical size of a letter could not normally exceed the em.
In digital type, the em is a grid of arbitrary resolution that is used as the design space of a digital font. Imaging systems, whether for screen or for print, work by scaling the em to a specified point size.
In digital type, the relationship of the height of particular letters to the em is arbitrarily set by the typeface designer. However, as a very rough guideline, an "average" font might have a cap height of 70% of the em, and an x-height of 48% of the em.[citation needed]

Alternative definition

One em was traditionally defined as the width of the capital "M" in the current typeface and point size,[1] as the "M" was commonly cast the full-width of the square "blocks", or "em-quads" (also "mutton-quads"), which are used in printing presses. However, in modern typefaces, the character M is usually somewhat less than one em wide. Moreover, as the term has expanded to include a wider variety of languages and character sets, its meaning has evolved; this has allowed it to include those fonts, typefaces, and character sets which do not include a capital "M", such as Chinese and the Arabic alphabet. Thus, em generally means the point size of the font in question, which is the same as the height of the metal body a font was cast on.

Not really sure of the history of it.

[gwpriester]

It is from the days of hot metal type (before computer type), and as sketch has researched is a unit of measurement. There are also em dashes — and en dashes – which are the width of an M and an N.