(C-x means ctrl+x, M-x means alt+x)
The default prefix is C-b. If you (or your muscle memory) prefer C-a, you need to add this to ~/.tmux.conf:
| FILE SPACING: | |
| # double space a file | |
| sed G | |
| # double space a file which already has blank lines in it. Output file | |
| # should contain no more than one blank line between lines of text. | |
| sed '/^$/d;G' |