” Man is the measure of all things” Protagoras (c.485-c.410 BCE)
Archaic measurements, certainly inherited from Asia, are mentioned in both Homer and Hesiod: the dactyl, palm, foot, cubit, span, and fathom, all corresponding to different parts of the human body. As we move forward in time, and east to west, we find the rule and square based more on the foot than the cubit (the distance between elbow and the tip of the hand). The Greek cubit is divided into sixteen dactyls (fingers). The Roman folding rule was based on the Italic foot, and divisible into four palmae, twelve unciae (inches), and sixteen digiti. The carpenter’s square was indispensable for laying out joinery, rafters, and stairs. Some had a blade and a tongue of different lengths, to be used to indicate different standards. More often, the two arms were of a ratio of three to four, making the hypotenuse measure a multiple of five, an accurate and easily checked right angle. It is curious, then, that the Roman square is of two equal lengths of two feet, a triangle whose hypotenuse is an irrational number (√2) and thus more difficult to represent accurately. This square was known as the norma, from whence we have the English words “norm” and “normal”. A right angle was formerly called a ‘normal angle’. The instrument was made from three wooden rulers (regula) pegged together. Two arms were held in place by a diagonal piece measuring two-foot ten inches, which is very close to a right angle, but not perfect. But it was perfect enough for the Roman builder (the diagonal is only six hundredths of an inch too long) and allowed him to attach a plumb-bob and lay the long side horizontally to act as a level. These squares were collapsible and portable and therefore widely distributed by the Roman legions
Mysteries of the Carpenter 