Any building with more than a handful of doors eventually faces the key chaos problem: supers carrying janitor rings of thirty keys, managers handing out copies of everything, nobody sure who can open what. A master key system replaces chaos with hierarchy.
How the hierarchy works
Each lock is pinned to accept its own individual key plus the keys above it in the hierarchy. A tenant's key opens their apartment. A floor sub-master opens every apartment on that floor. The building master opens everything. Managers carry one key; tenants carry one key; everyone opens exactly what they should.
Where it pays off
Brownstones converted to rentals, small apartment buildings, multi-room offices, retail with stockrooms — anywhere one organization controls many doors. Even a five-door office benefits: the manager stops carrying five keys, and turnover means rekeying one cylinder, not five.
The danger: cross-keying
Badly designed systems suffer cross-keying, where the wrong key opens the wrong door — usually from sloppy pinning math. This is why master keying is a design discipline, not just a pinning job. We chart every system on paper, calculate the pinning to eliminate cross-keying, and physically test every key against every door it should and shouldn't open.
Key control completes it
Pair the system with a restricted keyway and unauthorized duplication becomes impossible — the system's integrity can't be undermined by a trip to the hardware store. Keep the keying chart secure, log who holds which keys, and expand through your locksmith so new doors slot into the existing hierarchy cleanly.