1401. Two cases, the accusative and the ablative, are used with
prepositions.
1402. Prepositions were originally adverbs which served to define more
exactly the meaning of a verb.
Thus, #endo#, _in_, _on_, the older form of #in#, is an adverb, in an
injunction occurring in a law of the Twelve Tables, 451 B.C., #manum
endo iacitō#, _let him lay hand on_. Similarly, #trāns#, _over_, in
#trānsque datō#, _and he must hand over_, i.e. #trāditōque#.
1403. In the course of time such adverbs became verbal prefixes; the
verbs compounded with them may take the case, accusative or ablative,
required by the meaning of the compound. Thus, #amīcōs adeō#, _I go to
my friends_ (1137); #urbe exeō#, _I go out of town_ (1302).
1404. For distinctness or emphasis, the prefix of the verb may be
repeated before the case: as, #ad amīcōs adeō#; #ex urbe exeō#. And when
it is thus separately expressed before the case, it may be dropped from
the verb: as, #ad amīcōs eō#; #ex urbe eō#.
1405. The preposition thus detached from the verb becomes an attendant
on a substantive, and serves to show the relation of the substantive in
a sentence more distinctly than the case alone could.
1406. A great many adverbs which are never used in composition with a
verb likewise become prepositions: as, #apud#, #circiter#, #īnfrā#,
#iūxtā#, #pōne#, #propter#, &c., &c. The inflected forms of
substantives, #prīdiē#, #postrīdiē# (1413), #tenus# (1420), and #fīnī#
(1419), are also sometimes used as prepositions. And #vicem# (1145),
#causā#, #grātiā#, #nōmine#, #ergō# (1257), resemble prepositions
closely in meaning.
1407. A trace of the original adverbial use of prepositions is sometimes
retained, chiefly in poetry, when the prefix is separated from its word
by what is called _Tmesis_: as, #īre inque gredī#, i.e. #ingredīque#,
Lucr. 4, 887, _to walk and to step off_. #per mihī̆ mīrum vīsum est#,
_DO._ 1, 214, _passing strange it seemed to me_.
1408. Even such words as are used almost exclusively as prepositions
sometimes retain their original adverbial meaning also: as, #adque
adque#, E. in Gell. 10, 29, 2, _and up and up_, _and on and on_, or _and
nearer still and still more near_. #occīsīs ad hominum mīlibus
quattuor#, 2, 33, 5, _about four thousand men being killed_. #susque
dēque#, _Att._ 14, 6, 1, _up and down_, _topsy turvy_, _no matter how_.
1409. On the other hand, some verbal prefixes are never used as separate
prepositions with a substantive. These are called _Inseparable
Prepositions_; they are: #amb-#, _round_, #an-#, _up_, #dis-#, _in two_,
#por-#, _towards_, #rē̆d-#, _back_. Usually also #sēd-#, _apart_ (1417).