Augustus' chief concern as a stylist was clarity of expression. To achieve that end he was willing to employ prepositions where their use might, strictly speaking, have seemed redundant. In 50 b.c. Cicero had self-consciously apologized to Atticus for a similar practice. Seven years earlier, among the regulations governing a temple of Jupiter, prepositional phrases occur in place of the expected dative case. The increasing prominence of prepositions in marking syntactic relations previously expressed by case inflection represents one of the most important grammatical developments in the history of Latin.

In addition to presiding over the demise of the inherited case system, Latin prepositional usage has special interest for its exemplification of diverse syntactic and semantic change. Prepositions reflect the expression of time–space relations in the Roman world and play a central role in the construction of idioms. Notable semantic realignments have been occasioned by the interaction between prepositional and prefixational uses of the same form, or of different forms with similar or opposite meanings.

Like other Latin prepositions, prae and pro are commonly discussed in one of two ways. Traditional lexicographers distinguish between ‘literal’ and ‘figurative’ usage, isolating a basic meaning to which all other meanings are presumably subordinate. Thus, according to Lewis and Short, prae is used ‘literally’ when it refers to place, but ‘figuratively’ in comparisons.