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Add or remove border left

Windows shortcut 
AltL
Mac shortcut 
⌘⌥←

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 left border added or removed from the border preview area.

On the Mac, this shortcut works directly on the worksheet, and adds a left border to each cell in the selection.

222 Excel Shortcuts for Windows and Mac