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# Rulers
You can overlay horizontal and vertical rulers on a web page:
```{image} viewport_rulers.png
:class: center
```
The units are in pixels.
The dimensions of the viewport are displayed near the top-right corner of the viewport.
To 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.
```{image} settings_available_buttons.png
:class: center
```
Once 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.
```{image} ruler_toggle_button.png
:class: center
```
Behavior to keep in mind when using rulers:
- The rulers command must be reapplied in **new tabs** and after **each page refresh.**
- The command isn't permanent.