n. a location in a cylinder at which specific tumbler surfaces must be aligned, removing obstruction(s) which prevented the plug from moving

The dividing line between the plug and the shell (the height to which the tops of the lower pins must be raised to open the lock).

The interface between the plug and shell in a cylinder that is normally obstructed by the pin tumblers. The pins must be raised to the shear line by the correct key to allow the plug/key to turn.

The area where the top surface of the plug and the cylinder housing meet. The height which the bottom pins must be raised by the key in order to rotate the key cylinder. Usually a cylindrical shaped tumbler which is often bullet shaped and comes in a variety of lengths that correspond to the depth of the cut of the notch in the key.

In lock picking a cylinder lock, the shear line, also known as the split line in Australia, is where the inner cylinder ends and the outer cylinder begins. When a correct change key or master key is inserted in the cylinder, it will align the pin segments with the shearline and allow the cylinder to be turned. This break in the lock mechanism is a vital part of the lockpicking process, as it allows a picked pin to "hang" while the others are being picked.