A more thorough explanation:

Definition: Ulna ferrea is a historical measuring device made of iron that was approximately one yard in length. It was kept in the Exchequer as a standard measure.

Example: In medieval England, the ulna ferrea was used to measure cloth and other textiles for trade and taxation purposes.

The ulna ferrea was an important tool for ensuring fair trade and taxation in medieval England. It was kept in the Exchequer, which was the treasury of the English monarch, and was used as a standard measure for cloth and other textiles. This helped to prevent fraud and ensure that everyone was using the same measure. The ulna ferrea was also used in other parts of Europe and was an important precursor to modern measuring devices.