Note the notch shown here on the end of the wood coming off the roller: That is the rabbet.
Adam dry-fits the frame around the piece, in this case a 19th Century slate chalkboard, to make sure it fits.
Various lengths of wood
Shown here are various lengths of wood reclaimed from demolitions, renovations and barns. These pieces are ready to be cut into frames.
This barn wood (from real barns in the western Pennsylvania area) has been ripped into lengths and is ready for rabbeting.
More barn wood
Note how the nail and worm holes in this wood will add character to the finished frame.