What does Somme en toutes lettres mean?

"Somme en toutes lettres" is a French phrase that means "amount in words". It's often used in formal documents, such as checks, invoices, and contracts, to prevent fraudulent alterations of the numeric amount.

For example, if the amount is "123.45", the "Somme en toutes lettres" would be written as "Cent vingt-trois euros et quarante-cinq centimes".