CSS

/* class="first second" */
.first.second {}

/* class="first" OR class="second" */
.first, .second {}

/* class="first second", or class="second", or class="third second", or class="second third" */
.second {}

/* apply to any .child at any depth under .parent */
.parent .child {}

/* apply to .child if it is DIRECTLY under .parent */
.parent > .child {}