Knob cylinders are cylinder locks that are operated with a knob on the inside of the door. This allows them to be locked and unlocked quickly and easily without using a key. Various types of knob cylinders are available, including:

1. Single knob cylinder - This is a simple cylinder with a single knob on the inside of the door. The door can be opened or locked by turning the knob.
2. Double knob cylinder - This is a cylinder with a knob on both sides of the door. This allows the door to be locked or unlocked from both sides using a button.
3. Half knob cylinder - This is a cylinder with a knob on the inside of the door and no keyhole on the outside of the door. This allows the door to be locked and unlocked from the inside with the button and you have no way to open the door from the outside.
4. Push button knob cylinder - This is a cylinder with a push button on the inside of the door. This allows the door to be locked and unlocked without using a key. You have to hold the button and turn it to lock or open the door.
5. Electronic knob cylinder - This is a cylinder with an electronic lock that can be operated using a smartphone, code, card or fingerprint. This allows the door to be opened and locked quickly and safely.

Choosing the right knob cylinder depends on your specific needs and the level of security you wish to achieve. When choosing a knob cylinder, consider the quality of the lock, its security features and its compatibility with your door.