Abstract: A key-operated, combination changing, lock comprising a lock box, a slidable bolt and at least one bolt-retaining slider movable transversally across the bolt to retain the bolt in a resting and locking position, a bolt-locking arrangement being provided to hold the bolt once in the locking position. The bolt-locking arrangement comprising more than one locking plates capable of being actuated by at least one step of the key and being also capable of changing the combination of the lock when closed by a key different to the one used to lately open the lock. A key is also provided to operate the combination changing lock, having at least one bit step to actuate over the bolt-locking arrangement of the lock.