The ruler tool is a simple tool for quick measuring of sizes of screen objects (in pixels.) After invoking the command a ruler similar to the one shown below appears:
The ruler can be freely dragged over the screen simply by dragging any of its points. To close the ruler tool and to return back to the program press <Esc>.
The ruler's "hole" contains a hidden functionality. When clicked, it pops up a local menu containing the following options:
Enables the horizontal ruler (default). Shortcut: <Ctrl+H>.
Enables the vertical ruler. Shortcut: <Ctrl+V>.
Closes the ruler tool. Shortcuts: <Ctrl+F4> or <Esc>.