How to switch from input mode into command mode in ed ?

As far as I know, ed starts in command mode. Once I input i,a or c ed changes into input mode. So far it worked. Now I read that in order to switch from input mode into command mode one has to input "," and enter. I tried so, but it did not work, I stayed in input mode and had to quit the program with ctr+z without the opportunity to write the text in the buffer onto hard disk.

I am using linux mint and xface terminal, how can I switch into command mode ?