These are manufactured in 2ft X 2ft squares and the edges fit together like a mortise and tenon joint.
From the en.wikipedia.org
The tenon is cut to fit the mortise hole exactly and usually has shoulders that seat when the joint fully enters the mortise hole.
From the en.wikipedia.org
At the edge of one stack of timber is a squared piece of wood with a hand-cut mortise and tenon, the traditional joint used in many Japanese buildings.
From the usatoday.com
The open wound of the leg's stump reveals how it was originally connected to its host with a mortise and tenon joint, of the same kind used three millennia later in French cabinetry.