The top crosspieces support the seat - it is very important to make sure that they have horizontal tops when the legs are splayed to the correct angle...
Once the two frames were finished, a 12mm steel rod was cut in two, trimmed to the right length and inserted into each of the matched-up pivot holes. A small leather disk washer was placed between the two frames (you can just see it in the picture to the right) and the pivot holes were finished with a thin disk of wood glued over them to keep the rods in place. I was concerned over wear and tear in the soft pine at these points, but many years on, all is still well: indeed, I often stand on this seat to fix candles and the like above head-height in the pavilion. It's completely solid.