Counter
The white shape inside a letter, like the hole in an 'o' or the enclosed space in an 'e'. Small counters make text clog up.
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What it is
A counter is the white space enclosed, or partly enclosed, by a letterform. The centers of 'o', 'b', 'd', 'p', and 'q' are closed counters; 'c', 's', and 'n' have open counters; 'e' has both, a closed eye on top and an open lower counter reached through its aperture. Counters are not empty leftovers. They are drawn as carefully as the black strokes around them, and the name is literal: a punchcutter sank the interior into the steel punch with a smaller punch called a counterpunch before cutting the outside shape.
Open, generous counters help small text survive. That is why text faces and screen fonts often give 'e', 'a', and 's' more breathing room, while a display face can afford tighter, more dramatic shapes. When a word looks clogged at 14px, inspect the counters before blaming the weight alone.
Gotcha: counter and aperture are related but not interchangeable. The counter is the interior white shape; the aperture is the opening that connects an open counter to the surrounding space. Adding tracking moves letters apart but cannot reopen a counter trapped inside a glyph.
Ask AI for it
Build an HTML and SVG specimen that explains counters. Set lowercase 'a e o p' at 180px with CSS 'font-family: Georgia, serif', place the same letters beside a bold display face, and fill each enclosed white shape with a contrasting SVG path or mask. Label the interior shape 'counter' and label the opening in the 'e' 'aperture'. Include a 16px comparison underneath to show which counters remain open at reading size. Do not change letter-spacing between the two specimens.