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 Ctrl+Z without the opportunity to write the text in the buffer onto hard disk. I am using Linux Mint and the Xfce terminal, how can I switch into command mode?