Answer:
There is no legal or regulatory requirement for a second dollar amount in words. It's there on most checks generally due to custom, and of course, is used as a control to make alteration of the intended dollar amount more difficult.
The amount written out in words is sometimes referred to as the legal amount of the check. That's a misnomer. It's the legal amount only if it differs from the amount in numerals, when the amount in words controls.
First published on BankersOnline.com 9/28/09