prepositional phases can also function as post-head modifiers (that is, modify prepositions that precede them). Pre-head modifiers Adverb phrases can function as pre-head modifiers in prepositional phrases. For example, the prepositional phrase after midnight can be modified by adverb phrases such as shortly (shortly after midnight) or quite obviously (quite obviously after midnight). A subset of adverb phrase modifiers of prepositions express degree and occur within prepositional phrases but not other phrasal categories. These degree adverbs include clear, flat, plumb, right, smack, and straight. Examples of prepositional phrases modified in this way include clear up the tree, straight out the door, and right out of the park. Noun phrases indicating spatial or temporal extent can occur before a preposition that expresses spatial or temporal meaning in order to modify it. For example, the prepositional phrase beyond the post office can be modified by the noun phrase two miles (two miles beyond the post office) or a few minutes' walk (a few minutes' walk beyond the post office). Certain prepositions with directional meanings can function as pre-head modifiers in prepositional phrases. The prepositions down, out, over, and up frequently occur in this role. For example, the preposition down can modify the prepositional phrases by the beach (down by the beach) and by the sea (down by the sea). We can tell that these directional prepositions are modifying other proportional phrases rather than taking prepositional phrases as complements because the other preposition determines whether the whole phrase is grammatical. Thus, "I placed it up on the shelf" is grammatical because "I placed it on the shelf" is also grammatical, but "I placed it up to the attic" is not grammatical because "I placed it to the attic" is not grammatical. Post-head modifiers Prepositional phrases can also modify prepositions that precede them. In the clause they go out in the cold, for example, the preposition out is modified by the prepositional phrase in the cold. Though it may appear that in the cold could be modifying the verb go rather than the preposition out, movement of the elements to different parts of the clause suggests that in the cold is actually linked with the preposition out: the prepositional phrase in the cold cannot move to the start of the clause by itself (*in the cold they go out) but it can move to the start of the clause as part of the larger