"Simplex locks are keyless locks that have a sequence of buttons. You press the proper combination of buttons, and the lock opens. A typical lock has five buttons. A combination might be set to three buttons, and then you need to press them in the same order. You never press the same button twice."

For example the lock for the restroom at the Rob Hill campground in the San Francisco Presidio is opened by the combination 425, as I figured out by trial and error. (They probably changed it by the time you read this.)It is also possible to define combinations with four or five buttons. Is that more secure? Would it be much better to have six buttons?