Viewing a slide master isn't currently possible in PowerPoint for the web. You can, however, view a slide master in the PowerPoint desktop app.
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To open the desktop app, at the top of the ribbon, select Editing > Open in Desktop App.
Don't have the PowerPoint desktop app? Try or buy Microsoft 365.
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On the View tab, select Slide Master.
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Select Close Master View when you're done.
For more info, see What is a slide master?
