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Quick, clean, and to the point

Add or remove border upward diagonal

Windows shortcut 
AltD

On Windows, this shortcut only works within the Format Cells dialog box, on the Borders tab. If you use this shortcut in Format Cells, you'll see a border added or removed from the border preview area.

On the Mac, there is no equivalent shortcut.

This is probably the best explanation I've ever read of an excel function, and I've read a lot! Thank you for making it so clear and easy to understand!!! :) - Marie
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