31.5.2 Multiple Lines of Comments

If you are typing a comment and wish to continue it on another line, you can use the command C-M-j or M-j (comment-indent-new-line). If comment-multi-line (see Options for Comments) is non-nil, it moves to a new line within the comment. Otherwise it closes the comment and starts a new comment on a new line. When Auto Fill mode is on, going past the fill column while typing a comment causes the comment to be continued in just this fashion.

To turn existing lines into comment lines, use the M-x comment-region command (or type C-c C-c in C-like buffers). It adds comment delimiters to the lines that start in the region, thus commenting them out. With a negative argument, it does the opposite—it deletes comment delimiters from the lines in the region.

With a positive argument, comment-region duplicates the last character of the comment start sequence it adds; the argument specifies how many copies of the character to insert. Thus, in Lisp mode, C-u 2 M-x comment-region adds ‘;;’ to each line. Duplicating the comment delimiter is a way of calling attention to the comment. It can also affect how the comment is indented. In Lisp, for proper indentation, you should use an argument of two or three, if between defuns; if within a defun, it must be three.

You can configure C Mode such that when you type a ‘/’ at the start of a line in a multi-line block comment, this closes the comment. Enable the comment-close-slash clean-up for this. See Clean-ups.