Q: I have two questions regarding a tabletop board on a bookcase.
First, why use the split tenon/mortise design as opposed to a continuous tenon and mortise? Second, wouldn’t gluing the tenon into the mortise in the way shown prevent normal contraction and expansion needed for the center board?
A: I’ve been making breadboard ends for the better part of 20 years. The configuration and methods I use are quite similar to those that were shown to me by my late friend and former associate, Paul Otis Lee, who was an alumnus of the North Bennet Street School in Boston, Massachusetts. Paul is the person who validated my split tenon technique that you are asking about. It is interesting to me that some techniques — and breadboard ends are a perfect example of one of them — break all the rules of logical woodworking. With that said, however illogical they may be, breadboard ends have proved themselves practical again and again, and they’ve stood the test of time. On top of that, they look really good. So the answer to your first question is: because I learned it from a good friend and an excellent woodworker. Add to that I’ve done it that way for almost 20 years, and it works well.
The second question about glue and wood movement ... the answer is this: nothing prevents wood movement. You can slow it down, you can limit its effects, but wood is a dynamic substance. That means it’s going to move. For that reason, one rule I like to follow is this: “Don’t skimp on the glue! Glue is your friend.” Now, what I did for the top of the pine bookcase is arrange my joinery so that any noticeable movement — that inevitable seasonal expansion and contraction — would move toward the rear of the project, to avoid a noticeable projection at the front of the piece.