I’ve been using em-based text-sizing on this site for a number years using Richard Rutter’s methods and have done well by them. He recently expanded upon his tricks for A List Apart and proved the consistency you can achieve with them.

But there’s one thing that’s occasionally in the back of my mind: what exactly is an em? I had some remembrance that it originally got its name from some aspect of the size of a capital M, but that’s about it.

I must have glossed over Richard’s definition having read the article too many times:

“Classically, an em (pronounced emm) is a typographer’s unit of horizontal spacing and is a sliding (relative) measure. One em is a distance equal to the text size.”

Oh, that’s right, it’s the height of the font. Actually, there’s more to it than that, especially depending on whether you’re a typographer, a type designer, or a software engineer. Font Beureu’s Type 101 blog has the full details (including illustrations) in their post The Em. Go read it.

I’ll leave you with the following excerpt (and many thanks directed at Grant Hutchinson for noting the article):

“In my view, the em is a fundamental unit of typography. It plays a critical role in the design of a typeface, in the technology to compose and render the typeface, and finally in the decisions made by the typographer when setting the type. In fact, from the type designer’s point of view, the em is what forms the basic module used to compose letters into words, words into lines, and lines into paragraphs. It makes movable type possible.”