What is a exhibit appropriate tone for a business letter?

The term 'exhibit appropriate tone' for a business letter means to use a level of formality or informality that the suits the nature of the business communication for the specific letter.

However, even if you know the person receiving the letter very well, too much informality is not appropriate when communicating a business matter because the letter needs to be clear and concise to accomplish the goal of the letter; the recipient of the letter may have other people read and follow through with the letter; the letter can be used as documentation (proof) of what transpired in a communication.