I'm not sure how LESS handles it, but it adds your class to the selectors for that ruleset. If you do .myclass { @extend .container; }, you end up with .contain

I'm not sure how LESS handles it, but it adds your class to the selectors for that ruleset. If you do .myclass { @extend .container; }, you end up with .container, .myclass { // container styles }. It only adds minimal overhead.