Carbon copying, often abbreviated to c.c., is the technique of using carbon paper to produce one or more copies simultaneously during the creation of paper documents. A sheet of carbon paper is sandwiched between two sheets of paper and the pressure applied by the writing implement (pen, pencil, typewriter or impact printer) to the top sheet causes pigment from the carbon paper to make a similar mark on the copy. 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. Care must be taken to ensure that each sheet of carbon paper has the carbon side down.
The use of carbon copies declined with the advent of photocopying and electronic document creation and distribution (word processing). Carbon copies are still used in special applications, for example, in manual receipt books which have a multiple-use sheet of carbon paper supplied, in order that the user can keep an exact copy of each receipt issued, although even here carbonless copy paper is often used to the same effect.
It is still common for a business letter to include, at the end, a list of names preceded by the abbreviation "cc:", indicating that the named persons are to receive a copy of the letter, even though carbon paper is no longer used to make the copies. The term "carbon copy" can be used in reference to anything that was a near duplicate of an original ("...and you want to turn him into a carbon copy of every fourth-rate conformist in this frightened land!", Heinlein, Stranger in a Strange Land).
In common usage, To field recipients are the primary audience of the message, CC field recipients are others whom the author wishes to publicly inform of the message, and BCC field recipients are those surreptitiously being informed of the communication.