The ‘property’ (Alber and Prince 2016) is the fundamental unit of analysis of the OT typology: It classifies languages both grammatically, in terms of ranking conditions called ‘values’, and phonologically, because a property value realizes a phonological ‘trait’ that all forms of the language must comply with.

Property families classify languages of independent OT typologies into the same classes. Within a language class, languages share features of the grammar, correlated with the same kind of formal, extensional effects. Consequently, across typologies, a single phonological contrast has multiple reflexes; this, despite the fact that languages of the same class are not related in any obvious way.

To highlight the scope of this result, a single property family predicts that the following contrasts are equivalent: whether a language parses every syllable into a foot, whether a language is fully quantity-sensitive, requiring stress on every ‘Heavy’ syllable, whether a language is ‘default-to-opposite’ for the positioning main stress.