Comparing phonology in spoken language and sign language reveals that core properties, such as features, feature categories, the syllable, and constraints on form, exist in both naturally occurring language modalities. But apparent ubiquity can be deceptive. The features themselves are quintessentially different, and key properties, such as linearity and arbitrariness, although universal, occur in inverse proportions to their counterparts, simultaneity and iconicity, in the two modalities. Phonology does not appear full blown in a new sign language, but it does gradually emerge, accruing linguistic structure over time. Sign languages suggest that the phonological component of the language faculty is a product of the ways in which the physical system, cognitive structure, and language use among people interact over time.