The gathered ends on the arms of this chair are the detail that takes the longest and looks the most casual, which is the usual relationship in upholstery.
A ruched arm end is made by drawing the covering into radiating folds that all converge at the centre of the arm scroll. The folds have to be even in width all the way round, and they have to arrive at the centre without bunching - which they will do, every time, unless the fullness is calculated first.
So our upholsterers divide the circumference of the arm end into an even number of folds, mark each one on the frame, and pin every fold in place before any is fixed. Only when all of them are pinned and the spacing is confirmed by eye is anything stitched down. Working fold by fold, fixing as you go, guarantees the last few folds have to absorb all the accumulated error.
The centre is then closed with a covered button, which does the same job as the rosette on a carved scroll - it hides the one point where a radiating pattern cannot resolve itself.