By Joseph A. McGeough
The unit of linear measure in the ancient world, the cubit, was simply the length from the elbow to the extremity of the middle finger. Although the cubit gave an order of magnitude, it was hardly a standard, and it varied widely in different times and places.
One of many royal Egyptian cubits had a length of 52.43 cm (20.64 inches). It was divided into seven palms (measured across the fingers, not the knuckles), making a palm almost three inches. Each palm was, in turn, divided into four digits of about three-quarters of an inch apiece. Thus, 1 cubit = 7 palms = 28 digits. On occasion, digits were subdivided into 10ths, 14ths, or 16ths.
The common rule of Egyptian masons and carpenters was made of wood, had a narrow cross section, and had one bevelled edge, with the two left-hand palms carrying the smaller divisions of digits. Some Egyptian rods were made of stone and used digits divided into 16ths. These may have been ceremonial rods or, perhaps, master gauges for calibration and comparison; their brittleness would make them unsuitable for the rough handling received by mason’s tools.
The Romans introduced folding rules of bronze in 30- and 15-cm (12- and 6-inch) sizes. These were probably “pocket” instruments for officials—too expensive to be used by ordinary craftspersons, who probably used plain strip rules.
Only scanty evidence exists that graduated rules were used in the Middle Ages and the Renaissance; plain straightedges seem to have predominated. In 1683 an English writer described foot rules as having 1/8-inch (0.32-cm) subdivisions. The folding rule, now made of wood, reappeared at the end of the 17th century.
Measurement was long characterized by great national and regional differences. Because every large city in Europe and most towns had a different but locally standard “foot,” rules with four different graduations (one on each face) were made.