Leg Splay

The compressive strength of wood typically exceeds its tensile strength by a factor of 5.  Thus the strongest way to position wood is to have the grain aligned parallel to the force it is resisting.  To apply this principle in a chair design would result in the legs being entirely vertical.  While this design is theoretically strong, it is also very unstable.  As a compromise, chair legs are typically splayed out at 3-7 degrees in order to gain stability without compromising too much of the wood's compressive strength.

(NCTM Standard: Measurement)