Before sending a letter, you can specify that "carbon copies" be sent to other than the main addressees. You can also send "blind carbons." (Recipients of your letter can read the addresses for the carbon copies, but not the addresses for the blind carbons.)
Many people send themselves carbons or blind carbons in order to retain a copy for their own record.
There are three methods for sending carbon copies with a letter:
~c hank@fretful george@lonesome stormy@snoozer
A Cc: line is also created by the ~h command, which displays the entire header of the letter. ~h prompts you with To:, Subject:, Cc:, and Bcc: (blind carbon copy) lines, one line at a time. Blank lines can be filled in; filled lines can be retyped. As with other tilde commands, always use the ~h command on a new line.
~c, ~h, and other tilde commands are described in "7.9 Tilde Commands".