Historically, the term "em" goes back to the days of printing with metal type, and referred to the width of a capital "M" in the current typeface and point size

Historically, the term "em" goes back to the days of printing with metal type, and referred to the width of a capital "M" in the current typeface and point size: https://en.wikipedia.org/wiki/Em_(typography) https://en.wikipedia.org/wiki/Em_(typography) There's also a unit called an "en", which was based on the width of "N": https://en.wikipedia.org/wiki/En_(typography) https://en.wikipedia.org/wiki/En_(typography)