Both in photography and in typography, the aperture describes an opening. In cameras, its size correlates with how much light goes in. In typography, it’s closely related to what’s called a counter.
As per Wikipedia, a counter is the area of a letter that is entirely or partially enclosed by a letter form or a symbol (the counter-space/the hole of).
Letters containing closed counters include A, B, D, O, P, Q, R, a, b, d, e, g, o, p, and q. Letters containing open counters include c, f, h, i, s etc.
The aperture then is the opening between an open counter and the outside of the letter.
There are also variations. Take lowercase ‘g’ for instance, which has two typographic variants. There is the single-story version which is much seen in sans serifs with one closed counter and one open counter (and hence one aperture). Then there is also a version more often seen in classic typefaces – the double-story which has two closed counters.
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