Prepositions (or more generally adpositions, see below) are a class of words expressing spatial or temporal relations (in, under, towards, before) or marking various semantic roles (of, for).Their primary function is relational. A preposition typically combines with another constituent (called its complement) to form a prepositional phrase, relating the complement to the context.The word preposition (from Latin: prae, before and Latin: ponere, to put) refers to the situation in Latin and Greek, where prepositions are placed before their complement and hence pre-positioned. English is another language employing them in this way. In many languages (e.g. Urdu, Turkish, Hindi, Korean and Japanese), they come after the complement. Thus applied, they are called postpositions. Similarly, circumpositions consist of two parts that appear on each side of the complement. The technical term used to refer collectively to prepositions, postpositions, and circumpositions is adposition. Some linguists use the word "preposition" instead of "adposition" for all three cases.Some examples of English prepositions (marked as bold) as used in phrases are:Adpositions perform many of the same functions as case markings, but adpositions are syntactic elements, while case markings are morphological elements.