The shiplap is formed by overlapping rabbets cut into opposite faces of adjoining boards. I'd call it a car-penny kind of thing, used in siding and natural-wood paneling.
For example, in our shop here at Rodale. Fred cladded several walls with ship-lapped cedar. (The cedar is photogenic, and it's easy to hang things on—just drive a nail anywhere.) Rather than just butt the 1 by 12 boards edge to edge, he rabbeted the edges so each board could overlap its neighbor. Once in the warm, dry shop for a few weeks, the cedar dried further, and naturally it shrank pretty dramatically. The ship-laps prevented the real guts of the walls from being exposed.
And that's the whole point of shiplaps. The rabbets arc quickly cut—precision is irrelevant—yet they serve a valid purpose. A piloted rabbet bit and a router arc all you need to make them. l-ay the board across sawhorses and run the router along one edge, cutting the rabbet half the thickness (or a little more) of the stock. Roll the board onto its back and repeat.
The utility of the shiplap as a precision-Fitted joint? Noi much.
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