The function of your mortise lock could make a big difference. Just imagine what would happen if your lock doesn’t behave as you expect – at best you might get locked out and pay me $75 to unlock your door, at worst some burglar or other person with worse intentions could enter while you sleep, or a child could get locked inside a bathroom.
I am frequently asked to visit a particular building in Belltown to unlock doors. This is because doors in this building are locked either with the key or by using a toggle switch on the side of the door. This toggle switch can be manually operated, but it automatically changes to locked whenever the deadbolt is thrown with the key or the thumbturn inside. This is unexpected behavior. This function is called corridor (ANSI F13) and is unusual. I would go so far as to say it is a poor choice for a 20 story residential building though I have made a lot of money over the years thanks to the architect who made this choice decades ago.
A much more appropriate choice in functionality is dormitory, or ANSI F21. This requires a key to lock the door from the outside. The only way to get locked out with this type of lock is to lose the key. To be fair a significant number of lockouts in this building have been because people dropped their keys down the elevator shaft. If the architect who made choices for door hardware for this building had chosen dormitory locks it would have saved the future occupants tens of thousands of dollars.
Perhaps a more common problem are people with locking door knobs for their bathroom who lose the key. There is a place for keyed locks on bathroom doors. Here in Seattle if you don’t lock bathroom doors in downtown businesses people go in them to do drugs and sometimes they overdose and die.
This would be unexpected in a private home bathroom though where a privacy lock is much more appropriate. Despite this I am asked to unlock bathroom doors with keyed knobs on them at least once a month. I don’t pry into the circumstances of how this came to be when confronted with this situation. I offer to replace the lock with a privacy function lock and suggest that the owner themselves could also replace it.