On stick chairs I drive the wedges across the seat grain on 3° tapered tenons — who first put that in print as the “right” way? Also curious if any old shops made an exception for elm seats because of the interlocked grain, or was it always across, regardless of species?
I think the earliest clear ‘across the seat grain’ I saw in print was John Brown (echoed by @ChrisSchwarz), with the older Roubo logic of wedging across the receiving member’s grain behind it. On elm I still go across, but I skew the kerf about 5–10° toward the ring arc and test on a bored scrap first — its interlock can be as contrary as a cat in a bath. Do you saw your kerfs before glue-up or once the leg’s home?