A combination lock is a lock that opens when you turn a dial or press buttons in a specific sequence of numbers, rather than using a key. The sequence of numbers is called the combination. Many students use combination locks on their school lockers: you might turn the dial right to 15, left to 8, then right again to 23 to make it open.

The clever thing about combination locks is that only someone who knows the combination can open them. Without a key to lose or copy, a combination lock stays secure as long as you keep the numbers secret and don't share them with others.

The dial on a combination lock typically goes from 0 to 40. Some newer locks use buttons instead of a dial, working more like a keypad where you press 4, 5, 9, 2 in order. Either way, the principle is the same: the correct sequence opens the lock, while any other sequence keeps it firmly closed.

The word combination here means a specific arrangement of numbers that must be entered in exact order, which is why you can't just know the numbers. You also have to know the correct sequence to dial them.