The main components in the pin-and-tumbler design are a series of small pins of varying length. The pins are divided up into pairs. Each pair rests in a shaft running through the central cylinder plug and into the housing around the plug. Springs at the top of the shafts keep the pin pairs in position in the plug.
When no key is inserted, the bottom pin in each pair is completely inside the plug, while the upper pin is halfway in the plug and halfway in the housing. The position of the upper pins keeps the plug from turning -- the pins bind the plug to the housing.
When you insert a key, the series of notches in the key push the pin pairs up to different levels. The incorrect key will push the pins so that most of the top pins are still partly in the plug and partly in the housing. The correct key will push each pin pair up just enough so that the point where the two pins come together lines up perfectly with the space where the cylinder and the housing come together -- this point is called the shear line.
Some locks are designed to work with two different keys. The change key will open only that specific lock, while the master key will open that lock and several others in a group. In these locks, a few of the pin pairs are separated by a third pin. This third pin is called a master wafer or spacer.
When three pins are combined in a shaft, there are two ways to position the pins so they open the lock. The change key might raise the pins so that the shear line is just above the top of the master wafer, while the master key might raise the pins so the shear line is at the bottom of the master wafer. In both cases, there is a gap at the shear line and the key is able to turn.
In this lock design, the lowest pin is the same length in each lock in the group, but the master wafer varies in length. This lets the person with the master key access any lock in the group, while someone with a change key can open only his or her own lock.