You probably have one of these if you have a printer. It's basically the same mechanism as on the output paper guides, that causes a movement of the guide on on

You probably have one of these if you have a printer. It's basically the same mechanism as on the output paper guides, that causes a movement of the guide on one side to match the other side, and for the paper path to therefore remain centred. Normally you can take the output tray off and turn it over to see the mechanism.