What Is Counter?
A counter is the enclosed or partially enclosed white space inside a letter — the hole in an "o," "b," or "e," or the open space inside a "c" or "u." Counter size affects legibility: open, generous counters keep letters distinct and readable at small sizes.
Fully enclosed counters (in o, p, d) are called closed; partially open ones (in c, s, u) are called open. Their proportions strongly influence a font's texture.
When ink spreads or screens render small, generous counters keep letters from filling in and turning into blobs, which is why faces for small sizes open their counters wide.