Target elements with multiple classes, within one rule

Question

I have some HTML that would have elements with multiple classes, and I need to assign them within one rule, so that the same classes could be different within different cont

Answer 1

.border-blue.background { ... } is for one item with multiple classes.
.border-blue, .background { ... } is for multiple items each with their own class.
.border-blue .background { ... } is for one item where '.background' is the child of '.border-blue'.

See Chris' answer for a more thorough explanation.