Have you ever seen a contract where the number was both written and spelled out? Something like “purchaser hereby purchases fifty (50) cases of the product.”
Did you ever wonder why the number appeared twice? It seems to go against Bluebook Rule 6.2 (a) on the use of numbers.
Bryan Garner’s Legal Writing in Plain English provides an answer. The double number convention actually comes from a centuries-old attempt to avoid fraud. When modern printing presses were less common and most people copied text by hand, it was easy to add a few digits to a numeral. It was far harder to change both the numeral and the word. This process is still done on checks.
During the days of carbon copies, writing a number twice was sometimes helpful because sometimes only one number would make it to the bottom copies.
The convention proliferated during carbon copy times, which is why we see the double number so often in contracts.