---
name: Aperture
slug: aperture
category: typography
status: published
created: 2026-08-21T00:00:00.000Z
modified: 2026-08-21T00:00:00.000Z
definition: The partly enclosed opening in a letter, such as the gap where the
  two terminals of a c stop or the mouth at the bottom of an e.
aliases:
  - name: open counter
  - name: letter opening
tags:
  - fonts
relations:
  contrastWith:
    - legibility
    - counter
  variantOf: []
  partOf: []
  seeAlso: []
implementations: []
sources:
  - title: "Typography Terms: Glossary (NN/g)"
    url: https://www.nngroup.com/articles/typography-terms-ux/
  - title: A Glossary of Typographic Terms (Monotype)
    url: https://www.monotype.com/resources/typographic-terms
demo: inline
exhibit: false
useWhen: why one sans feels clearer at small sizes
---

An aperture is the gap where a letter stops short of closing. A `c` is a ring
with a piece removed, and the distance between the two terminals left behind is
its aperture. An `e` has one under the crossbar, an `a` and an `s` have two, a
`G` has one where the bowl meets the bar. Where the shape does close, the
enclosed white is a counter instead, which is why an `o` has counters and no
aperture at all and why "open counter" is a loose second name for the same
thing.

The size of that gap is a decision a type designer makes, and it separates whole
families of sans. Neo-grotesques in the Helvetica line close their apertures
hard: the terminals curl toward each other and the letters end up looking like
variations on the same ring. Humanist and legibility-driven faces cut the
terminals off at a steeper angle and leave the mouth wide open. At display sizes
the difference reads as personality. At the sizes real interfaces are set at, it
reads as whether the letter survives.

That is the practical stake. A narrow aperture is the first thing to disappear
when type gets small, low in contrast, or rendered onto too few pixels: the gap
fills in, the `c` becomes an `o`, the `e` becomes a blob, and
[legibility](/legibility) collapses exactly where a reader most needs it, in
codes and names and numbers that context cannot repair. It works the same way
under glare, at distance, and for readers with low vision, which is why
signage and instrument faces open their apertures further than anything drawn
for a page.

Aperture travels with two other measurements. A tall
[x-height](/x-height) buys room for a wide mouth, and a face that pairs a small
x-height with closed terminals has almost nothing left at small sizes. And
[optical size](/optical-size) is the same compensation drawn as an axis: the
small cut of a family opens its apertures precisely because the setting will
close them again.