Can you advice prefix key

Hi, I built up a menu based on the M-o prefix key, but maybe it is not the best one. Could you advice me a free or rarely used key to use as a prefix-key ?

Not directly what you asked but one thing i use a lot is the [keychord ](https://github.com/emacsorphanage/key-chord) : press two keys simultaneously like as, sd, df, etc. It enables you to add many more bindings without even using ctrl or meta. To use it as a prefix key instead of a single command bind the keychord to an [hydra](https://github.com/abo-abo/hydra) Like (key-chord-define-global "sd" #'my-hydra/body)

The “obvious” answer is C-c, since C-c followed by unmodified letters is reserved for the users custom bindings.

Depends on your particular keyboard. I used to have my custom keymap assigned to the Menu/App key in the MS Sculpt keyboard. Then I had a translation of Print Screen to Menu in Emacs for the Thinkpad keyboard (Print screen is where Menu *should* be). Nowadays I have a split keyboard and F6 is at the edge of the left side so I assigned that key instead. Look at your keyb and a few candidates will pop up, after trying them for a bit you will realize quickly if they are good or not. For a while I tried C-' and on paper it seemed ideal but it never clicked.

`C-z`

M-o is maybe a hard to hit, depending on the layout of your keyboard. I personally have define two extra prefixes: C-z and C-v. The rationale is that they are close to other prefixes, C-x and C-c. so I have four prefixes next to each other on my keyboard. [This is the layout of keyboard I use](http://daniel.haxx.se/blog/wp-content/uploads/2014/08/my-keyboard2.jpg).