The size of the opening where an open Counter meets the exterior of a letterform. It is visible in characters such as c, e, s, and a. Aperture ranges from wide open (as in many Humanist Sans Serif) to nearly closed (as in Neo-Grotesque Sans Serif designs like Helvetica). Wide apertures aid letter differentiation and small-size Legibility; narrow apertures contribute to a more uniform, contained texture. Aperture is one of the most reliable visual cues for distinguishing humanist from Grotesque Sans Serif and neo-grotesque sans serifs.