There were several cubits of different magnitudes that were used. In Egypt, the common cubit was the length of the forearm from the elbow to the tip of the middle finger (about 18 inches / 46 cm). It was divided into the span of the hand (one-half cubit), the palm or width of the hand (one sixth), and the digit or width of a finger (one twenty-fourth).
Because one person's forearm tended to be a different length to the next person's, a standardised Royal Master Cubit, or Sacred Cubit, was cut in granite. This was 7 palms or 28 digits long, and was used in the construction of buildings and monuments (such as the pyramids) and in surveying.
The inch, foot, and yard are thought to be connected to the cubit through a complicated transformation not yet fully understood. Some believe they evolved from cubic measures; others believe they were simple proportions or multiples of the cubit.
See also: Historical weights and measures