If you find yourself in a situation where you need to replace a key but don't have the original one, an experienced locksmith can help. Before you contact a locksmith, you'll need to provide certain information. To increase your chances of success, I recommend testing the keys on your locks with the door open. You can buy more keys online or visit a junk shop and get the corresponding vintage versions.
It's also possible to leave a key in each lock or on a hook next to the door. According to an old house guru I spoke to, there are three skeleton-type keys that account for approximately 90% of interior door locks. This works on a lot of different styles of locks, but the hardest part is figuring out which blank key is in the lock. When you have a blank key, it's easy because you're making a duplicate, so you have the blank piece in front of you and can copy it.