The em dash is as wide as the type is tall

The fact An em is a relative unit equal to the point size: at 12pt type, one em is 12pt. The letter M was simply the character that historically filled that square in metal type.

Why it matters Once you know an em scales with size, most of the mysterious spacing units in typography stop being mysterious. Tracking, indents and margins can all be set in ems and survive any change of size.