preposition (STR) = an adposition which occurs before the head of the referring phrase. Example: in on the table, on is an adposition that precedes the table. (Section 4.3)
"It is often claimed that prepositions are not a lexical category, but rather that they are simply case markers on noun phrases, possibly even inserted by transformations... These analyses are based on the mistaken assumption that the only possible complement to a preposition is NP; if prepositions enforce strict subcategorization restrictions and occur with such bizarre complements as PP and even NP - PP, the analysis is obviously untenable. Nevertheless, there is some relationship between prepositions and cases which we would do well to clarify at this point.
The reason for considering prepositions to be case markers is semantic: prepositions in one language often translate as case markers in another language, and vice versa. And even within a single language, prepositions and case markers often perform very similar semantic functions, as alternative realizations of indirect objects, for example." (Jackendoff 1977: 80)