A common pin tumbler lock only has one shear line. A cylinder with only one shear line provides less security than a cylinder with multiple shear lines. Multiple shear lines increase pick resistance, and depending on the design, also offers other benefits such as expanded master keying capacity.

Definition: Shear line – in a cylinder lock, the shear line is where the inner cylinder (the plug – what you put your key in to) ends and the outer cylinder (the shell) begins. The shear line is created when a key’s cuts align the tumbler pins in the cylinder to a point where there is an unobstructed separation of bottom and top pins that allow the plug to rotate within the shell. When a key with the wrong cuts is inserted into the cylinder, the bottom and top pins won’t align and either one of the two (bottom or top pin) will block the rotation of the plug.

Language to use in key system specification:
The manufacturer’s key system design shall at a minimum include two locking elements that will require two shear lines be established by an authorized key (traditional shear line for split pin tumbler cylinders/cores and a shear line established by a secondary mechanism).