I rarely use one much anymore. But on ordinary carpenter's squares there are many markings for use on various tasks. I recall that there are many-many measurements and calculations that can be done with one. But I don't know what they are. And, I suspect, most folks don't know either. This has to be a real old-time "Neanderthal" skill that is being lost.
Can anyone point to a source for information on what all can be done with a square?