1====== 2Rulers 3====== 4 5You can overlay horizontal and vertical rulers on a web page: 6 7.. image:: viewport_rulers.png 8 :class: center 9 10The units are in pixels. 11 12The dimensions of the viewport are displayed near the top-right corner of the viewport. 13 14To be able to toggle rulers for a page, you first need to enable the button by going to the settings page for the Developer Tools and checking "Toggle rulers for the page" under Available Toolbox Buttons. 15 16.. image:: settings_available_buttons.png 17 :class: center 18 19Once enabled, the "Toggle rulers for the page" button appears at the top right of the Toolbox, in the same place as the Settings/Options button. 20 21.. image:: ruler_toggle_button.png 22 :class: center 23 24 25Behavior to keep in mind when using rulers: 26 27- The rulers command must be reapplied in **new tabs** and after **each page refresh.** 28- The command isn't permanent. 29