The band of key fret under the seat is, technically, the easiest carving on this chair. It is also the one most likely to go wrong, because geometry does not forgive and foliage does.
A carved leaf has no correct answer. If a stem curves a little further than the drawing, nothing is broken - the eye accepts it as the carver's hand. That tolerance is what makes foliage forgiving and why apprentices start there.
Key fret has none of it. Every line is straight, every turn is a right angle, every gap is meant to be identical, and the whole band is repeated dozens of times in a row. An error of half a millimetre in one line is measurable against its neighbour, and once the eye finds one wrong corner it starts checking every other one.
So this band is not drawn freehand at all. The lines are gauged parallel from the edge of the rail, the divisions are stepped off with dividers so they come out whole, the corners are squared, and only then does a V-tool follow the lines. The carving takes an afternoon; the setting out takes nearly as long.
It is the part of the chair a carver is quietly judged on, precisely because there is nowhere to hide in it.