Prepositional phrases (PP) consist of a preposition and as a rule a noun phrase.  With regard to each other they fulfil the functions Relator and Prepositional Complement, respectively (see figure 5.1). 
Figure 5.1In terms of their internal structure prepositional phrases differ from noun phrases, e.g. in that preposition and `preposition Complement' are mutually dependent. On the one hand, in an example like (66) de cannot be omitted without causing the sentence to become ungrammatical.
(66) de vehiculo … dicebatThis rule does not hold for verbs like liberare, where an argument slot may be filled either by a noun phrase or by a prepositional phrase (see p. 67). On the other hand, as a rule a preposition cannot independently replace an entire prepositional phrase, a phenomenon which we will see with regard to so-called substantivised adjectives (p. 88). A prepositional Complement can only be omitted in cases such as (85):
(85) et in corpore et extra esse quaedam bona (`Some bodily and external things are good', Cic. Fin. 2.68) (see K.–St. I. 579)Extra does not only occur as a preposition, but also as an adverb. Its normal use, however, is as exemplified in (86):
(86) in urbe et extra eam (`Within the city and outside it')When two prepositional phrases governing different cases are coordinated, both prepositions require an explicit Complement. 
Pinkster, Harm (1942-) , Latin Syntax and Semantics [info], xii, 320 p.: ill.; 24 cm. [word count] [Pinkster].