ya, seriously.

The reason Bootstrap has a 12-unit grid (instead of 10, 16, etc..) is that 12 evenly divides into 6 (halves), 4 (quarters) and 3 (thirds). This makes adapting to a variety of layouts much easier. Bootstrap's grid columns are identified by different col-(breakpoint)-(units) CSS classes. So for example, col-md-3 would be a column that takes up 3 of the 12 units (or 25%) across in a row. I wrote an in-depth article on how the Bootstrap grid works , and Bootstrap 4 is a whole different animal because flexbox, and the new auto-layout columns which enables any number of columns across the viewport .