A definite noun phrase used to refer to a specific letter already known in context. It can be a mailed note, printed document, or an email. Use it when both sides know which message is meant.
For in every particular of the Word there is an internal sense which treats of things spiritual and heavenly, not of things natural and worldly, such as are treated of in the sense of the letter. Emanuel Swedenborg
The shelves of books we haven't written, like those of books we haven't read, stretches out into the darkness of the universal library's farthest space. We are always at the beginning of the beginning of the letter A. Alberto Manguel
Don't you, when strangers and friends come to call, straighten the cushions, kick the books under the bed and put away the letter you were writing? How many of us want any of us to see us as we really are? Isn't the mirror hostile enough? Jeanette Winterson
To do things wrong with best intentions is better than acting to the letter of the law with evil intentions. Jewish Proverb
The letter killeth, but the spirit giveth life. Romanian Proverb
The letter enters with blood. Spanish Proverb