It's (perhaps not?) well known that locks that are master keyed are inherently less secure than locks that aren't
It requires roughly half the picking effort.
in a lock you have multiple sets of pins. the key pushes pins, and if it pushes all the pins so that the top of the pin is at the boundary of the lock (the shear line), the lock turns.
There is a spring that pushes a connected pin down, which is what actually prevents the lock from turning. These are called driver pins. there is a separate pin(s) that the key actually interfaces with. The key pushes the pins until the driver pin moves past the shear line, when all driver pins and key pins are not interfering with the shear line, you can rotate/whatever the key and it is unlocked.
A master-keyed lock has additional discs inside the keyway, usually below the normal pins (closest to the key.) The discs are added based on the amount of extra movement needed to accept both the non-master, and the master key. So a master keyed lock has two, separate shearing points, the top of the regular pin, and the top of the master disc. This means there are at least two set-points for picking to get the driver pin out of the way - where the driver pin is flush with the shear line (as it would be with a regular, non-master key,) and where the normal pin's lower face is flush with the shear line (as it would be when a master key is inserted).
It requires roughly half the picking effort.
in a lock you have multiple sets of pins. the key pushes pins, and if it pushes all the pins so that the top of the pin is at the boundary of the lock (the shear line), the lock turns.
There is a spring that pushes a connected pin down, which is what actually prevents the lock from turning. These are called driver pins. there is a separate pin(s) that the key actually interfaces with. The key pushes the pins until the driver pin moves past the shear line, when all driver pins and key pins are not interfering with the shear line, you can rotate/whatever the key and it is unlocked.
A master-keyed lock has additional discs inside the keyway, usually below the normal pins (closest to the key.) The discs are added based on the amount of extra movement needed to accept both the non-master, and the master key. So a master keyed lock has two, separate shearing points, the top of the regular pin, and the top of the master disc. This means there are at least two set-points for picking to get the driver pin out of the way - where the driver pin is flush with the shear line (as it would be with a regular, non-master key,) and where the normal pin's lower face is flush with the shear line (as it would be when a master key is inserted).