(available from the Dock icon, too)
Opens a horizontal ruler. This ruler can be changed into a vertical ruler at any time (click the according button, see next image):
A ruler can be closed with this button:
The size of a ruler can be changed with the resizer:
Define the transparency of a ruler in the Settings
dialog.
Opens a vertical ruler.
Opens a ruler for both height and width of a layout.
All faded rulers will be shown again.
All visible rulers will be faded.
Closes all opened rulers.