A CodePen by Christopher Bloom.

style.scss

@import "compass";
// an extendable class
.extendable-style {
  font-weight: bold;
  line-height: 1.7;
}

// apply
.thingy-1{ @extend .extendable-style; }
.child .of .something{ @extend .extendable-style; }

// output
.extendable-style,
.thingy-1,
.child .of .something{
  font-weight: bold;
  line-height: 1.7;
}