Perpendiculum

Perpendiculum (κάθετος, μολυβδίς, στάθμη), a plumb-line, a string with a piece of metal attached, used by masons, carpenters, &c. to test the correctness of their perpendicular lines (Vitr. 7.3, 5; Isid. Or. 19.18): hence the expression ad perpendiculum of the correct line (Cic. Ver. 1.51, 133; Caes. Gal. 4.17, &c.). Cicero (ad Qu. Fr. 3.1, 2) distinguishes it from linea (=κανών), the line for measuring horizontally. This linea was called in Greek also (σχοῖνος and σπαρτίον, and, from its being coloured to make a mark, μιλτεῖον. Blümner, wrongly we think, excludes στάθμη from this sense, and makes it altogether equivalent to perpendiculum. There is no doubt that it was sometimes a plumb-line, as in Anth. Pal. 6.103, (στάθμην μολιβαχθέα: but that it was also (perhaps more commonly) a horizontal line is clear from its use to make a straight τάφρος in Hom. Od. 21.121. It was probably a line getting the true direction either way; and the expressions παρὰ στάθμην, ἐπὶ στάθμην would come from either use. We have also the phrases πρὸς κάθετον, εἰς κάθετον = ad perpendiculum. (Blümner, Technologie, 2.235.)

[G.E.M]