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135.
Cardinals and Ordinals have the following uses:

a. In numbers below 100, if units precede
tens, et is generally inserted
duo et vígintí;
otherwise et is omitted: vígintí duo.
b. In numbers above 100 the highest
denomination generally stand first, the next second, etc., as in English.
Et is either omitted entirely, stands
between the two highest denominations:  mílle (et) septingen sexágintá
quattuor, 1764.
NOTE: Observe the following combinations of numerals with
substantives: 

 únus et vígintí
mílités, or vígintí mílités (et)
únus, 21 soldiers.
 duo mília quíngentí
mílités, or duo
mília mílitum et quíngentí, 2500
soldiers.
 mílités mílle
ducentí trígintá únus, 1231
soldiers.
c. After mília the name of the objects enumerated
is in the genitive:

 duo mília hominum, two thousand men.[1][Or, in
poetry, bis mílle
hominés, twice a thousand men.]
 cum tribus m=ilibus mílitum, with three thousand
soldiers.
 m=ilia passuum tria, three thousand paces (three miles).


d. For million, billion, trillion,
etc., the Romans had no special word but these numbers were expressed by
multiplication (cf. § 138. a).

e. Fractions are expressed, as in English,
by cardinals in the numerator and ordinals in the denominator. The
feminine gender is used to agree with pars expressed or understood: 
twosevenths, duae septimae
(sc. partés);
threeeighths, trés
octávae (sc. partés).
 Onehalf is dímidia
pars or dímidium.
 NOTE 1: When the numerator is one, it is omitted
and pars is expressed:
onethird, tertia pars;
onefourth, quárta pars.
 NOTE 2: When the denominator is but one greater than the
numerator, the numerator only is given: twothirds, duae partés; threefourths,
trés partés, etc.
 NOTE 3: Fractions are also expressed by special words
derived from as, a pound: as, triéns, a third; bés, twothirds. See § 637.