Abstract: A lock system has a master-only lock which can be opened only by a master key and a subordinate lock which can be opened by a subordinate key and a master key. The master-key slot of the master-only lock is formed with a plurality of parallel longitudinally extending and inwardly projecting guide ridges and a plurality of parallel longitudinally extending and inwardly open variable recesses interleaved with and parallel to the guide ridges. The master key is complementarily shaped. The subordinate lock has guide ridges and variable recesses identical to that of the master-only lock, and in addition is formed with further variable recesses in which fit corresponding further ridges on the subordinate key. Thus, the subordinate key is of larger cross-sectional area than the master key so that it cannot fit into the master lock.