A prepositional phrase is an adverbial or adjectival phrase comprised of a preposition and a noun or other substantive that acts as its object. It may also contain modifiers. Prepositional phrases describe relationships between words in a sentence.
The composition of a prepositional phrase can be quite simple by including only a preposition and a noun, as in "at home." Often, however, the phrase also contains adjectives that modify the noun, such as "my" in "at my home," or "my most magnificent" in "at my most magnificent home." To complicate matters slightly, the object of the preposition might not even be strictly a noun, but might be another substantive like a pronoun or gerund, such as the gerund "bathing" in the phrase "after bathing." If the object is a pronoun, it is always in the objective case, as in "before me" rather than "before I."
The prepositional phrase may also have a compound object, meaning more than one substantive joined by a coordinating conjunction.In English, the coordinating conjunctions are "and," "but," "for," "nor," "or," "so" and "yet." For example, a prepositional phrase might have two objects joined by "and" like "among the trees and bushes." A similar construction might even be used with a negative formation: "in the snow but not the ice." In this phrase, "snow" and "ice" are the objects of the same preposition, "in."
The relationships most often described by prepositional phrases are spacial or temporal in space or time. For example, "between the two houses" expresses a spacial relationship, and "between noon and one" expresses a temporal one. Prepositional phrases can also express more abstract relationships, such as "under pressure," where "under" refers to a circumstance rather than a location.
Within a sentence, a prepositional phrase can function either adverbially or adjectivally. In other words, it can tell either how something happens, or it can describe a noun. For instance, "The kitten with brown eyes lay with a peaceful expression" contains both types of prepositional phrase. "With the brown eyes" describes the noun "kitten," so it is adjectival. The words "with a peaceful expression" tell how the kitten lays, making it an adjectival prepositional phrase.