A cylinder lock (1) has a standard lock mode (12 o'clock position) and a service lock mode (10 o'clock position). If the lock plug is turned to the service lock mode with the standard key (8) and left in this position, the plug can be rotated with the aid of the service key (9). The service key is latched against withdrawal, when the lock is in its standard mode. The plug can be displaced axially through a distance of about 0.9 mm, after inserting the standard or service key. A latching device (22) prevents axial return movement of the plug when an attempt is made to remove the service key with the lock in its standard lock mode. This can avoid unintentional jamming of the lock in the standard mode.