The -xs- infix was removed from Bootstrap 4 Alpha 6, so the classes are simply, col-1, col-2.. col-12, etc...
Since Bootstrap is "mobile first", the xs breakpoint (<576px) is the default and are no media queries for it. There are only media queries for the larger breakpoints to override the smaller breakpoints. Starting with the smallest each breakpoint overrides the next..
xs(default) > overridden by sm > overridden by md > overridden by lg > overridden by xl
