Andy Dingley wrote:

It was somewhere outside Barstow when Duane Bozarth
wrote:

If the two surfaces are planed and parallel to each other, making an
edge perpendicular to one face also makes it perpendicular to the
other...

Only in geometry - this is woodworking.

If the two surfaces are initially parallel, having gone through a
properly set thickness planer, working on the edges doesn't change that.

If there's twist and one resurfaces one surface, then it's true one will
have destroyed the symmetry, but that's a different operation.