Before publishing your site, you may want to preview what it's going to look like when it's live. Preview mode will hide the design studio UI and allow you to see your site as it will appear to visitors on the web.

To open preview mode, select it from the View menu, or press "P" on your keyboard.

While you're in preview mode, the design studio UI will be hidden and a bar across the top of the screen will indicate that you're currently in this mode. (The bar is only visible to you while you are working in the studio; it will not be visible on your live site once it has been published.)

To exit preview mode, click "Exit Mode" at the top right of the screen.
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At the top right of the preview, you'll see a series of buttons representing different device types:

Selecting one of these will allow you to preview the page at different screen sizes: 1280x768, 1024x768, 768x1024, and 375x559. These are the preset sizes that TIME Sites uses to represent tablet (landscape), tablet (portrait), and phone (portrait), respectively. 

You can read more about Device Modes in this article.