An interface is defined as a point of contact—a borderline—between domains of description (Phonetics, Phonology, Morphology, Syntax, Semantics), which are themselves conventionalized interfaces/borderlines. This approach to interfaces as borderlines is applied to the word-class of Prepositions. The paper addresses the debate concerning the status of P as a Lexical (open-class) versus Functional (closed-class, grammatical) category. P is analyzed as a Lexical category that has the status of the elsewhere case. The author shows that most Ps have the semantic, syntactic, morphological, and phonological properties of Lexical categories. The few Ps that have the minimal properties of function words are argued to be light prepositions (akin to light verbs). This analysis of P as a Lexical category departs from the literature on language processing, language production, language change, L1 acquisition and L2 acquisition, which is virtually unanimous in treating P as a Functional category. The data is drawn mostly from English, with some examples from French.