Palmus

Palmus (also palma, Plin. Nat. 7.28), properly the width of the open hand, or, more exactly, of the four fingers, was used by the Romans for two different measures of length; namely, as the translation of the Greek παλαστή, or δῶρον in old Greek, and σπιθαμὴ respectively. In the former sense it is equal to 4 digits, or 3 inches, or 1-4th of a foot, or 1-6th of the cubit, (Varro, R. R. 5.1; cf. Col. 5.1; Frontin. Aq. 24.) This was the only sense in Latin of the best age, but a later sense appears (first in ecclesiastical writers, Jerome, Ezech. 40, &c.), in which palmus=σπιθαμή, a span of 9 inches. It is a mistake to suppose that this measure existed earlier in Latin as palmus major. The Romans had no special word in earlier times for σπιθαμή, but expressed it as dodrans ( 3/4 of a foot): ternas spithamas, hoc est ternos dodrantes (Plin. Nat. 7.26). In the passage sometimes quoted from Varro, R. R 3.7, the ordinary palmus of 3 inches is meant. (Hultsch, Metrologie, p. 15, note.)

[P.S] [G.E.M]