In a cylinder lock

Cylinder lock

A cylinder lock is a lock constructed with a cylinder that a locksmith can easily unscrew to facilitate rekeying. The cylinder may contain any of a variety of locking mechanisms, including the pin tumbler lock, the wafer tumbler lock and the disc tumbler 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 break in the pin is reached by picking, the pin will "hang" at the shearline due to the space between the inner and outer cylinder. This "imperfection" in the lock mechanism is an unavoidable defect in the manufacturing process that allows for lock picking

Lock picking

Lock picking is the art of unlocking a lock by analyzing and manipulating the components of the lock device, without the original key. Although lock picking can be associated with criminal intent, it is an essential skill for a locksmith...

. Lock picking is a vital part of the locksmithing process as it can grant non-destructive entry to the lock device thus allowing it to be rekeyed

Rekeying

Rekeying normally refers to the ability to change a lock so that a different key may operate it. Rekeying is done when a lock owner may be concerned that unauthorized persons have keys to the lock, so the lock may be altered by a locksmith so that only new keys will work...

instead of being replaced.