What do the letters "CC" at the bottom of a letter stand for?

Before the development of photographic copiers, a carbon copy—not to be confused with the carbon print family of photographic reproduction processes—was the under-copy of a typed or written document placed over carbon paper and the under-copy sheet itself. When copies of business letters were so produced, it was customary to use the acronym "CC" or "cc" before a colon and below the writer's signature to inform the principal recipient that carbon copies had been made and distributed to the parties listed after the colon. With the advent of word processors and e-mail, "cc" is used as a merely formal indication of the distribution of letters to secondary recipients.

A sheet of carbon paper is placed between two or more sheets of paper. The pressure applied by the writing implement (pen, pencil, typewriter, or impact printer) to the top sheet causes pigment from the carbon paper to reproduce a similar mark on the copy sheet(s). More than one copy can be made by stacking several sheets with carbon paper between each pair. Four or five copies is a practical limit. The top sheet is the original and each of the additional sheets is called a carbon copy.