This joint is known as the tongue and fork joint. Here is an exploded view of two 4×6 rafters at 12/12 pitch with 1 1/2″ tenon set 1 1/2″ off the reference side with a 3/4″ oak peg.
To ensure a proper fit, these rough-sawn rafters have also been reduced to the next smaller 1/2″ in size at the joint locations.