This frame and panel back is in the vise for jointing the edge and dialing-in a good fit to the end panel. The groove receives a spline which serves to align the parts during assembly. The photo below shows the back of the left end panel on top of my pencil layout where I align the parts while fitting.
Fitting these backs and end panels requires several passes of the hand plane. Off of the jointer (the machine, that is) I'm able to get the panels roughly square. However, as shown below, there is still a bit of a gap. The ruler tick marks are spaced 1/32" apart and I estimate the gap in the joint to be very close to the width of the tick marks, which I, in turn, estimate to be just over 0.006". That leaves me with about a dozen or so passes of the hand plane to whittle that down to a good fit.
Here's the array of panels fitted and aligned with splines.
After the final surface preparation with a smoothing plane, and edge softening with a block plane, it's finally time for finish.
The last couple of weeks have been especially challenging logistically as I find resting places for these 6 panels, and alternately apply coats of shellac on the interior surfaces and oil on the exterior. Two sets of saw horses and the top of my table saw are the staging areas for finishing.
Below is one of the backs with a fresh coat of oil on the frame.
Next, two light coats of beeswax polish and the glue-ups will begin.
Hej då and happy shavings!