The em is an important and basic thing in typography, but its definition can be quite tricky. If you ask a type designer, a typographer, and a software engineer to define the em, you will probably get three different answers. The answers will not necessarily contradict each other; they are just from different points of view.
the typographer’s em
Let’s start with probably the most common usage of the term ‘em’, what I will call the typographer’s em. You probably encountered it early on in your typographic education.
The typographer’s em, a horizontal space, is equal to the type’s point size, a vertical measure. So the em space for a 10 point font would be 10 points of horizontal space. The em for a 48 point font would be 48 points of horizontal space. The font’s point size, not its design, determines the size of the typographer’s em. Every 10 point font has a 10 point em.
When setting type the typographer uses the em to gauge horizontal spaces within a paragraph. For example, there is the en space which is half the em, and the thin space which is a quarter em (or a fifth of an em space, depending on who you ask). These spaces are used for tasks like indenting the first line of a paragraph and setting tables. The em is also used to gauge different kinds of punctuation, such as the em dash, which is sort of like an elongated hyphen, and the en dash which is, predictably, half the width of the em dash.
(I hate to bring this up because I am afraid it will just confuse things. However, in fact many contemporary type faces don’t contain true em or en dashes. They look too wide. This is a break with typographic convention. But you still understand what the typographer’s em is, right?)
the type designer’s em
Similar to the typographer’s em, the type designer’s em is also connected to point size. However, in contrast to the typographer who uses the em as a horizontal space, the type designer thinks of the em as a vertical space. It forms the design space in which a typeface is drawn. This design space is a uniform height throughout the typeface. Let’s call this the type designer’s em.
Here’s how it works. When a type designer draws a typeface, the dimensions of the typeface are sized so the letters will fit within a single vertical measure, the type designer’s em. It’s not an absolute measurement, but a designated area where the letters can be drawn. The height of the typeface’s em is determined, in part, by the distance from the bottom of the typeface’s lowest descender to the top of its highest ascender. Plus, there is usually a bit of additional height added as a buffer. Then, all the characters and spaces that make up a typeface are drawn, aligned, and spaced as a proportion of the type designer’s em. The result is that each character’s body has a uniform height regardless of the shape of the actual character.
The concept of the type designer’s em was used back in the old days and it is still used today. In metal type, pretty much everything had to fit within the physical height limitations of the type designer’s em. Every letter had to fit within the block of metal, or body, it was cast on. In digital type, these physical limitations don’t exist. However, most glyphs in most digital typefaces, especially textfaces, still generally conform to the top and bottom boundaries of the em. This isn’t just because of tradition. By working within type designer’s em, type designers help type setting systems set type with enough space between the lines.
the software engineer’s em
If you ask a software engineer about the em, he would probably have a few things to add to what we’ve already said so far. I am going to simplify but this next part is still going to be technical.
The software engineer’s em is similar to the type designer’s em in that it’s the consistent vertical space each character of the font fits within. The difference is that the software engineer sees the type designer’s em as a grid consisting of fractional units, or em units. The shape and spacing information are plotted according to these em units. Each curve and point are located on this grid.
Usually an em unit is 1/1000th of the em. This means the height of the body for every character in the typeface is 1000 em units. The widths, which vary depending on the width of the character, are also expressed in em units. This is the basis for the font’s spacing information, or metrics. These are starting point for a glyph’s position during digital composition, the amount space between the characters, and the space between the lines. The type setting system combines the font’s metric information with input from the typographer about point size, tracking, and leading, to compose the lines of type with the spacing intended by the type designer.
The context is important when you define the em. To the typographer the em is a kind of space you can use to indent the first line of a paragraph. To the type designer, the em is the design space where a typeface is drawn. To the software engineer, the em is a dimensionless unit for plotting a typeface’s shape and spacing information.
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.
Not all typefaces, all type setting systems or all software will conform exactly to the way I have described things here. You might find typefaces with characters that exceed the limits of the em, software that uses additional ways to determine spacing, and different vocabulary than I have used. You can build on this basic foundation with additional knowledge.
This was a very long explanation for a just a single two-letter term. And now, you can safely forget most of it without affecting the way you use type. Thanks to automated typesetting, you can set type without ever having a good understanding of what the em really is. In fact, many authors have demonstrated you can even write a good book about setting type without ever really explaining what the em really is. But you’ve taken a closer look and now you have deeper understanding of what’s going on when you set type.
Finally, there is one last big thing to clear up. The em and the letter ‘M’ are pronounced the same in English. However, any further relationship between the em and the letter ‘M’ is just coincidence.