.order-0 to .order-5 set the flexbox order property; .order-first is order: -1 and .order-last is order: 6. Columns with equal order keep source order, which is why the second row of the previous listing reads C, B, D, then A. Every order class takes infixes: order-last order-md-first puts a sidebar under the article on phones and before it on desktops.