If the keel edges of the garboard planks are left with a sharp corner they will be hard to keep aligned as they are stitched. To solve this problem a chamfer, or small bevel, is planed in these two edges.
Lay one of the garboard planks on the table with the side with the gains facing down. Holding a sharp plane at a 45 deg. angle to the face of the plank, plane a shallow bevel the full length of the keel edge of the plank. The bevel should be about 1/8" wide and 1/8" deep. Repeat with the other garboard plank.