I recently got a Logitech Marble Mouse at work. It’s a nice little trackball but it has a minor problem. There is no button 2. “What is button 2?” you might ask. Button 2 is the middle mouse button. On your average scroll mouse, this is the button that is triggered when you click the scroll wheel. “So?” you might then ask. So, the middle mouse button is used in X to paste stuff from the X clip board.