Great question — answering as the UphillTech team:
Updated 2020... Bootstrap 5 In Bootstrap 5 (alpha) there is a new -xxl- size: col-* - 0 (xs) col-sm-* - 576px col-md-* - 768px col-lg-* - 992px col-xl-* - 1200px col-xxl-* - 1400px Bootstrap 5 Grid Demo Bootstrap 4 In Bootstrap 4 there is a new -xl- size, see this demo . Also the -xs- infix has been removed , so smallest columns are simply col-1 , col-2 .. col-12 , etc.. col-* - 0 (xs) col-sm-* - 576px col-md-* - 768px col-lg-* - 992px col-xl-* - 1200px Bootstrap 4 Grid Demo Additionally, Bootstrap 4 includes new auto-layout columns . These also have responsive breakpoints ( col , col-sm , col-md , etc..), but don't have defined % widths. Therefore, the auto-layout columns fill equal width...
Full answer by Carol Skelly: https://stackoverflow.stackexchange.com/a/19865627 (CC BY-SA 3.0).
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