This article contributes to a better understanding of the syntax-phonology interface. It offers a prosodic trigger for extraposition which accounts for the following asymmetry: While extraposition of subject, adjunct and attributive clauses is optional in German, object clauses must appear in the right periphery of the clause. It is argued that the constituents following an object clause in its preverbal base-position cannot be a parsed into phonological phrases. Such a configuration causes a defective prosodic clause structure. This deficiency is resolved by extraposition, which derives a str.