Going through some of my woodworking files I found
a diagram from Lee Valley that illustrates - on two
axis - how a piece of wood will change dimensions
as it shrinks, based on where it was cut from the
log. Shows why quarter sawn boards are so stabile.
If you understand what a board wants to do you can
sometimes do things to discourage it from doing so.
Wood moves. Plan for it.
diagram in a.b.p.w.