When printing technology was primarily based on printing inked metal types on paper, ink could easily spread in the small corners of the characters (in printing, this effect is called bleed), especially at small sizes, weakening their legibility.

One of the best examples of a typeface solving that problem is Bell Centennial, designed by Matthew Carter in 1975 for the US telephone company AT&T which needed a typeface for their phone books (printed on a thin and porous paper). This typeface has inner corners to go into the letterforms’ usual contours, called ink traps.

In digital typeface design, designers still use ink traps, especially for typefaces intended for small sizes (on printed and/or digital media), but also as design features (which can go pretty wild!).