Side bit

A side bit is a cut on the side of a key blade — perpendicular or angled to the standard top-edge cuts — that engages a side-bar mechanism inside the cylinder. Side-bit keys require both the standard depth cuts (which lift the bottom pins to the shear line) and the side-bit cuts (which release a side-bar) to be present and correct before the cylinder rotates.

The side-bar is a small horizontal bar inside the housing that, by default, protrudes into a groove on the plug. As long as it’s protruding, the plug is locked even if the pin-tumbler shear line is correctly aligned. The side-bit cuts on the key push pins (or rotate fingers) that retract the side-bar, allowing rotation.

Side-bit profiles in commercial use

Several restricted-keyway platforms use side bits:

• Schlage Primus — finger pins read the side-bit cuts; UL437 listed.
• EVVA EPS — two side-bars, two sets of side-bits.
• ASSA Twin — side-bit cuts plus standard pin-tumbler.
• Medeco Original — angled bottom-pin cuts that double as side-bit reads (rotation, not just lift).

Each implementation differs mechanically but the security property is the same: you can’t make a working duplicate just by reading the standard depth cuts. Side bits are typically tighter-tolerance than the depth cuts, and they’re often patent-protected.

Side bits and master keying

For the locksmith, side bits add a layer of complexity at the bench but not at the design stage. Keyzee allocates side-bit cuts alongside depth cuts during system design — the export pinning card includes both — and the engine validates that side-bits don’t generate phantoms within the side-bar mechanism (an analogous problem to depth-cut phantoms, but at the side-bar shear line).

Related

→ Keyway — the broader cylinder identity side-bits sit inside of → Shear line — what side-bars create a second of → Restricted keyway — most side-bit platforms are restricted