A "siliqua" (plural: siliquae) was the smallest Roman unit of weight measurement, being equal to 1/144 of a Roman "uncia" (ounce), of which there were twelve to a Roman "libra" (pound). Thus the siliqua was 1/1728 of a libra. The name was applied as early as 323 to the silver coins which the successors of the tetrarchy had begun to strike at a weight of 1/96 of a libra (theoretically 3.40 grams). By 324 the solidus (q.v.) was the Roman gold unit (struck at 1/72 of a libra - theoretically 4.55 grams) and it was tariffed at 24 of the silver coins. Hence the term "siliqua" was naturally applied to the silver coin, since it was equal to one-twenty-fourth of one-seventy-second of a pound of gold, or 1/1728 of a pound of gold (1/24 x 1/72 = 1/1728).
Popular usage has assigned the name "argenteus" to the earliest tetrarchic silver issue, although without ancient basis. Some of the early issues carried "XCVI" as the reverse design, which clearly means that they were struck at 96 to the Roman pound. Yet although the tetrarchic silver coins were struck in about 95% fine silver, their weight at the earliest averaged only about 3.15 grams, as against the 3.40 grams to be expected if they were truly struck at 96 to the pound (ignoring the slight debasement). This "over-tariffing" is common among precious metal coins, since the coined metal commanded a premium over bullion because of its "state-certified" weight and purity.
When the term "siliqua" was applied the weight peak of siliquae remained about 3.15 grams, but about thirty-five years later, between 355 and 360, Constantius II reduced the weight of the siliqua to about 2.00 grams. Between about 363 and 392 some silver coins were struck at the earlier standard of 96 to the pound, but modern usage has given the name "1½ siliqua" or "Heavy siliqua" to those pieces which were struck alongside the "reduced" standard coins. Although the weight of the siliqua varied over its lifetime, its diameter remained constant at 18-20 millimeters (including the early argentei).
"3/4 Siliqua" (plural: 3/4 Siliquae) is the modern name for rare silver coins struck only at Treveri between 378 and 383 for Gratian, Valentinian II, and Theodosius I. The reverse design is always a phoenix standing left on a globe.
The weights of these coins vary from 1.3 to 1.78 grams and are thus much too light for the siliquae (see Siliqua) of the time (about 2.0 grams), but too heavy for the half-siliquae which averaged slightly over one gram. Thus they have been called "3/4 siliquae" since their average weight of about 1.5 grams is three-quarters of the usual 2.0 gram weight of contemporary siliquae. Some modern authorities have referred to these coins as "half-argentei", since their weight (about 1.5 grams) corresponds to about half of the weight (about 3.1 grams) of the silver coins struck under the Tetrarchy and now called "argentei" (singular: argenteus).
"1½ Siliqua" (plural: 1½ siliquae) is a modern term sometimes applied to the later Roman silver coins which were struck at the post-Diocletianic reform standard of 96 to the Roman "libra" (pound), or about 3.40 grams. Although the Tetrarchy theoretically struck a silver "argenteus"at 1/96 of a pound, in practice their silver coins were generally around 3.15 grams, and that weight declined slightly to about 3.00 grams under Constantine I. About 355 under Constantius II (337-361), the successor to the argenteus, the siliqua, was reduced in weight to about 2.00 grams. The name "siliqua" was a Roman unit of weight equal to 1/144 of a libra, or about 2.26 grams. Although the later, or "reduced" siliqua of Constantius II roughly corresponded with the weight suggested by its name, the name was attested as early as 323 when the weight of the siliqua was still about 3.10 grams.
In any event, certain "heavy siliquae" (based upon the earlier 1/96 libra standard) were again struck from about 363 until about 392 in both the Roman east and west. Although their weight was simply approximately that of the pre-355 siliquae, they are today called "heavy siliquae" or "1½ siliquae" in comparison with the bulk of the siliquae then being struck at the reduced weight of about 2.00 grams.