Shear line

The shear line is the horizontal boundary between the cylinder plug (the rotating part) and the housing (the fixed part) in a pin-tumbler lock. When a correct key is inserted, every pin stack in the cylinder splits exactly at the shear line — bottom pins entirely below it, driver pins entirely above it — and the plug is free to rotate.

When no key is inserted, the springs push driver pins down across the shear line into the plug’s chambers, jamming rotation. When the wrong key is inserted, at least one pin straddles the shear line — some part of a bottom pin sits above it, or some part of a driver pin sits below it — and the plug catches.

Why shear-line behavior matters for master keying

In a mastered cylinder, the shear line is more interesting. Adding a master pin between the bottom and driver pins creates two heights at which the chamber’s stack splits cleanly at the shear line — one when the change key is inserted (lifting the bottom pin to a specific depth), one when the master key is inserted (lifting the bottom pin to a different depth, but the master pin makes the new total height land at the shear line).

This is also what makes phantom keys possible. If a chamber has two valid splits, six chambers have 2⁶ = 64 combinations of valid splits. Only two of those combinations are intended — the others are phantoms.

Side-bar shear lines

Some high-security cylinders (Medeco, EVVA EPS, Mul-T-Lock) use a side-bar in addition to the standard pin-tumbler shear line. The side-bar requires a separate alignment — typically by rotating bottom pins, by lifting a finger pin, or by reading a side-bit cut on the key blade — before the plug can rotate. This adds a second logical shear line that has to be cleared simultaneously with the first.

Related

→ Pin tumbler — the mechanism the shear line is part of → Bottom pin — the pin below the shear line when correct → Driver pin — the pin above the shear line when correct → Phantom key — what happens when too many shear-line combinations align