The term carbon copy refers to an outdated method of producing an instant copy of a typed or handwritten document. By placing a sheet of carbonized paper between two documents, whatever was typed or written on the top sheet transferred to the bottom sheet by virtue of the waxy carbon between them being impressed upon the underlying paper.
A carbon copy was the easiest way to make a duplicate contract, application, sales receipt or other note. However, it had its drawbacks. Carbon paper was messy, and making more than one copy meant putting additional carbon sheets between subsequent sheets of paper. Additionally, after the carbon sheet was used once, it was generally thrown out, resulting in a lot of waste. Re-using carbon paper could result in poor copies.
Another drawback of a carbon copy was that the carbon sheet itself became a duplicate of the transferred material in reverse, when looking at the sheet carbon-side up. By holding the sheet to the light and reading through the back, the transfer could easily be read. This was especially problematic for governmental bodies dealing with sensitive information, but also became an issue when credit cards were used at point-of-sale transactions. Numbers and signatures were so often stolen that it became habit for customers to ask for the carbon sheet back. Electronic credit card “swipe” machines eventually took the place of hard-copy receipts, eliminating the need for carbon copies at point-of-sale.
Today a carbon copy is rarely used. Copy machines, once expensive, have become affordable and ubiquitous, built into the smallest office machines. In those cases where a carbon copy might still be handy, such as for repairmen in the field, carbonless copy paper made by chemical processes provide copies without intervening carbonized sheets. One example can be seen in personal checkbooks that create a carbon copy without carbon paper.
Perhaps oddly, the enduring legacy of the carbon copy is its initials: c.c. These are still used at the end of correspondence to indicate when copies are filed or forwarded to other parties. In email headers, the “c.c.” field is used to enter a second addressee in order to send him or her a "carbon copy” of the original email.