On more recent versions of Mac OS X there is a setting which prevents ScreenRuler from spanning over multiple monitors. To fix this, you will need to switch a setting off in System Preferences:
- Click on the Apple logo in the top left of your screen
- Select System Preferences
- Select Mission Control
- Uncheck Displays have separate Spaces
- Log off or restart your computer
- Launch ScreenRuler – the ruler window will now span over all your displays