Two chairs that stand side by side have to agree with each other in a way a single chair never has to, and the place it shows is the crest.
Each crest is carved by hand, so no two are ever identical - and on a facing pair, small differences at the top of a tall chair are read immediately, because the eye compares them against each other at the same height.
So our artisans carve both crests at the same bench, at the same time, working a stage on one and then the same stage on the other rather than finishing one and starting the next. The two develop together, and each is checked against its partner at every stage rather than against a drawing.
The buttoning grids are then set out from a single rod that carries both chairs' dimensions, so the diamonds line up across the pair when the chairs stand together. It is the same discipline used for a matched set of anything - do the pieces in parallel, not in sequence, because a copy made later is never quite a copy.