Column Gutters

I’m noticing a few of my columns on a full-width layout are slamming right up against each other. I can give them some padding and that works fine, but if I want anything resembling a border (I’m using extra CSS classes for that), they’re still abutting without gutters, and if I add a margin to a column to create one, it seems to be shoving neighboring columns onto the next line, which is not what I want to do, obviously.

What am I missing?