Sometimes it’s convenient to have a ruler on your Emacs screen. Not often but sometimes. For example, it can be useful for lining things up in artist-mode. Emacs has a very nice ruler that, among other things, tracks were the point is on the line.
You can toggle it on and off with 【Meta+x】 ruler-mode. Often times you (or at least I) want to turn it on for just a second to check something so you can turn it on and then type 【Meta+x Meta+p】 to bring the command up again to turn it off.
This isn’t a command you will use a lot but when you need it, it comes in very handy. If you forget its name, 【Ctrl+h f】 ruler will bring up the on-line documentation.