Ch unit

1ch is the advance width of the digit zero in whatever font is in effect on that element. Not the average letter, not the widest one: the zero, specifically, and the advance rather than the ink, so the sidebearings are included. The unit exists because the constraint typographers actually care about is stated in characters. Measure is a count of characters per line, so max-width: 66ch writes the rule down in the units of the rule itself, and the column then tracks any change in type size for free.

The reason it is the zero and not something more representative is practical: in almost every font the digits share one advance, so the zero is the most stable single glyph to hand to a layout engine. In a monospace face that makes ch exact, and 40ch holds exactly forty characters of anything. In proportional text it makes ch an approximation, and a generous one, because a zero is wider than an average lowercase letter and much wider than a space. A 66ch column of English prose typically fits closer to 75 characters, which is why about 60ch usually lands on the measure you were aiming for.

The unit is font-relative and font-specific, which is the part that surprises people: it is not just proportional to the type size like em, it also changes with the family. Set the same 45ch in two faces at the same size and you get two different pixel widths, because their zeros are drawn to different widths. So a column sized in ch survives a size change intact and does not survive a font swap intact, and a webfont arriving after first paint can visibly resize a column that was laid out against the fallback. Newer siblings fill the gaps for other axes: cap and ex for vertical measurements, lh for a line’s own height, and ic for the advance of a full-width CJK glyph, where a zero says nothing useful.