After a woodworker has cut his/her joinery, prior to final assembly they'll do a dry fit to make sure everything fits correctly. This can be challenging depending upon how complicated the piece is; the more tenons and mortises there that need to be lined up, the more swearing you'll do during the process.
That's why this swear-free video of a team of Japanese master carpenters is so stunning. Not only is the joint insanely complicated and huge, designed to connect two timber-frame beams end-to-end, but they're using a traditional method of tapered pegs to smoothly connect—and disconnect—the pieces:
Not complicated enough for you? Check out this dry-fit where two beams are joined inside of a third piece:
While the craftsmen obviously do most of the carving with hand tools—no machine exists that can cut those kinds of stepped mortises—some commenters on the video erroneously believe they use only hand tools. But watch that last video and pay close attention, at 2:34, to the lower edge of the screen: You can see the mortise bears the clear marks of having been initially hogged out with a Forstner bit. That's really the only indication I see that these men are mortals.