What is the difference between the signs for 'lock' and 'key'?
While they are related, 'key' is typically signed by holding an imaginary key with an 'X' handshape and twisting it into the open palm of your non-dominant hand. 'Lock' uses two full 'S' handshapes (fists) stacked on top of each other.
Can I use this sign for a combination lock?
Yes, this sign is used for the general concept of locking something, regardless of the specific lock type. However, if you are specifically describing the action of dialing a combination, you might mime turning a small dial with your fingers instead.
Does it matter which way I twist my hand?
Yes, the standard movement is to twist your dominant hand forward so the palm ends up facing down. Twisting it backward might look like you are trying to unlock it, though the actual sign for 'unlock' usually involves twisting and pulling the hands apart.