Compression (Wood Under Load)

Compression is the stress a piece of wood experiences when a force squeezes its fibers together along their length, the mechanical opposite of tension (a force pulling fibers apart). A loaded horizontal beam experiences both at once in different zones: the top face, curving slightly downward under load, is in compression, while the bottom face, stretching as the beam bows, is in tension -- which is exactly why beam and joint failures often start at the tension face first, since most woods (and most joints) tolerate compression noticeably better than they tolerate being pulled apart.

Understanding which forces a given joint or structural member is actually under -- compression, tension, or the racking/shear forces of being pushed sideways -- is what separates joinery chosen for looks from joinery chosen because it's actually suited to the load it'll carry.