DeMorgan’s Laws

These can come in handy when refactoring boolean logic.

Or more plainly in code:

!A && !B == !(A || B)
!A || !B == !(A && B)

These equivalences can be used to refactor your code to make things easier to understand. It’s not a huge change and it won’t rock your world, but every once in awhile using one of these substitutions will improve the code.