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Carbon copying, abbreviated cc or c.c., is the technique of using carbon paper to produce one or more copies simultaneously during the creation of paper documents. With the advent of email, the term has also come to refer to simultaneously sending copies of an electronic message to secondary recipients.
A sheet of carbon paper is placed 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, from the use of the carbon paper.
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 copies of the letter, even though carbon paper is no longer used to make the copies.
An alternative etymology is that "c:" was used for copy and "cc:" indicates the plural, just as "p." means page and "pp." means pages. This alternative etymology explains the frequent usage of "c:" when only one recipient is listed, while "cc:" is used for two or more recipients of the copies. This etymology can also explain why, even originally, "cc:" was used to list recipients who received typed copies and not necessarily carbon 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).
Use as a Verb 
Carbon copy can be used as a transitive verb with the meaning described under E-mail below related to the CC field of an e-mail message. That is, to send the message to additional recipients beyond the primary recipient. It is common practice to abbreviate the verb form, and many forms are acceptable, including cc and cc:. Past tense forms in use are CCed, cc'd, cc'ed, cc-ed and cc:'d. Present participle or imperfect forms in use include cc'ing. One authoritative source uses cc, cc'd and cc'ing, respectively.
In e-mail, the abbreviation CC indicates those who are to receive a copy of a message addressed primarily to another. The list of CCed recipients is visible to all other recipients of the message. An additional BCC (blind carbon copy) field is available for hidden notification; recipients listed in the BCC field receive a copy of the message, but are not shown on any other recipient's copy (including other BCC recipients). It is considered good practice to indicate to the other recipients that a new participant has been added to the list of receivers (e.g. by writing "I have CCed Notre Dame Center Braxston Cave").
In common usage, the 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.
Dot matrix and daisy wheel printers are also able to use carbon paper to produce multiple copies of a document in one pass, and most models feature adjustable impact power and head spacing to accommodate up to three copies plus the original printout. Usually, this feature is used in conjunction with continuous, prearranged perforated paper and carbon supplies for use with a tractor feeder, rather than with single sheets of paper, for example, when printing out commercial invoices or receipts.