Exceljet

Quick, clean, and to the point

Add or remove border bottom

Windows shortcut 
AltB
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 bottom border added or removed from the border preview area.

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

I LOVE your site. It helps me frequently as I have no formal training but I'm very logic oriented and this site is put together so very well and has assisted me so very much! You've improved the quality of my life. -Mike
Excel foundational video course
Excel Pivot Table video training course
Excel formulas and functions video training course
Excel Shortcuts Video Course
Excel Charts video training course