Functioning as a scalable typographic unit, an em is directly bound to the active typeface scale, meaning that a text size specified at 16 pixels establishes a single em as exactly 16 pixels.
The em square represents the foundational bounding box used to scale every individual glyph within a typeface. Consequently, when a font is rendered at 12 points, this underlying bounding area measures exactly 12 points across, with its upper and lower boundaries generally extending just above the capital letters and just beneath the lowest dropping character strokes.
The em dash derives its title from the concept that its horizontal span should equal one full em, even though numerous contemporary typefaces feature an em dash that is slightly more compressed. Meanwhile, an en dash occupies roughly 50 percent of the width allocated to an em square.
In the era of physical typefounding, the em was named because it matched the physical width of the uppercase letter M, but within modern digital font files, the capital M can be designed to be either broader or more slender than the actual em square of that typeface.
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