It is the most widespread. It works with a reversible flat key in sawtooth or with holes.
When the key is inserted, pins arranged perpendicularly to the axis of the rotor are aligned with the relief of the key and allow the rotation which causes the bit.
This one in turn drives the bolt.
The mechanism is simple, and easy handling... This type of cylinder is perfect for all people, including those who lack strength in the hands.
See also: the diagram of a round cylinder lock with pump.