Shear line (locksmithing)

In 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 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 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 instead of being replaced.

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Wikipedia