Length of the tenon is set by the remaining thickness of the sub top (or bottom)
With the simple dado complete, attention turns to the vertical divider. I don’t recall reading how to do this part anywhere, although I have read a lot and may have just forgotten the source. I say this as a disclaimer – I may not be doing this “right”, but it worked. The length needed for the tenon is marked with a gauge, just like dovetail joint thickness are marked. Set the gauge using the remaining thickness of the sub-top (or bottom). FYI this dado is about 1/8 inch deep.
Transfer that depth to the end of the vertical divider. Careful to mark the correct side. With hand prepped boards the sub top and bottom will not be exactly the same thickness.
Fix the vertical divider so you can work on the end. I highly recommend a Moxon style vise. Use a marking knife, pencil, and a square to mark out the tenons. No need to measure – do whatever looks right. I picked the width of my ruler as the width of my tenons. I use a knife to mark the end grain surface because the knife line helps guide the saw to get a straight square cut. Seriously, it helps – give it a try. Use a pencil to mark the vertical line on the long grain, it is just a visual guide to follow.
Critical step before sawing – use an “X” to mark the waste areas!!!!!
I used a backsaw for the vertical cuts and a bowsaw for the horizontal cuts to remove most of the waste between the tenons. Get as close to the baseline as possible so there will be less chisel work in the next step.
You may notice that I am blowing on the saw. When starting a new kerf I find it helpful to blow off the sawdust as it comes so I can keep my eye on the line.
The outside tenons are about 1 inch back from the edge. Those pieces are the easiest ones to remove as they only require turning the board on its side and making one saw cut. No chisel work required if you are handy with a saw.
If my positioning with the chisel looks awkward, it kinda is. I keep the chisel vertical during chopping by visually aligning the backside of the chisel with the line scribed to define the depth of the tenons. It is the easiest way I have found to consistently keep the chisel vertical without an extra guide.
After sawing between tenons, remove the rest of the waste with a sharp chisel and good mallet. This is just like chopping out dovetails, but easier with straight cuts. The tenons are now complete in the vertical divider. Time to make the mortices, but that will be the topic of Phase III.