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The concept of the shear line is crucial in the operation of pin tumbler locks because it is the specific horizontal plane where the top and bottom pins must be properly aligned for the lock to turn and unlock. When the correct key is inserted, the key raises the bottom pins to the correct height, allowing the shear line to be aligned with the top pins. This alignment is essential; if the pins are not aligned at the shear line, the lock mechanism remains engaged, preventing the cylinder from rotating and thus keeping the lock in a secured position.

Understanding the shear line is fundamental for locksmiths as it forms the basis for how they manipulate locks, troubleshoot issues, and carry out lockpicking techniques in order to gain entry when necessary. The focus on the shear line in the operation directly ties into the functionality of pin tumbler locks, differentiating between successful and unsuccessful unlocking attempts.