Carbon copy

Why are letters carbon copied?
Is a carbon copy a courtesy copy of a letter?

[Ouisch]

The term "carbon copy" dates back to the time when Xerox machines were rare and if you wanted to make multiple copies of a document you had to insert carbon paper between pages of typing paper.

The purpose of multiple copies is to send the letter/memo to several different people. For example, you are a department supervisor, and you have written a memo to the company president recommending that an employee gets a promotion. You would send a carbon copy of the memo to the employee so they could have a record of it for their personal file.