Top image: Copyright © RMN-Grand Palais (musée du Louvre) / Hervé Lewandowski.
The common cubit was the length of the forearm from the elbow to the tip of the middle finger – usually around 45.7 cm. The ‘royal’ cubit was slightly longer: a common cubit plus the width of the palm of the Pharaoh ruling at the time.
The single royal cubit master (primary standard) was a rod carved from a block of black granite. Surviving cubit rods are between 52.2 and 52.9 cm in length. Workers were then supplied with copies – cubit sticks made of wood or granite. The royal architect or foreman of each construction site was responsible for maintaining the accuracy of these.
At every full moon, the cubit sticks had to be brought to the royal cubit master and compared to it. Failure to do so was punishable by death. With this standardisation and uniformity of length they achieved amazing accuracy. The Great Pyramid of Giza is constructed with sides of 440 cubits (230.364 metres). Using cubit sticks, the builders were within 11.4 cm – an accuracy better than 0.05%.
Thus the basic ideas of modern calibration were born over 4000 years ago: common units of measurement, traceability, a hierarchy of standards and regular re-calibration intervals.