Extended type families are made up of several type families. They consist of typefaces that share essential characteristics while preserving their individual features.

For example, the ITC Stone type family consists of several subfamilies containing both Roman and italic in medium, semi-bold, and bold. All designs share the same cap height, lowercase x-height, stem weight, and general proportions.^[1]

The benefit of choosing an extended type family is having fonts for a variety of use cases. You don't need to find typefaces that don't clash, because extended type families have a very uniform, orderly mood and aesthetic.