Internal walls that support joists at mid span and transfer loads down to foundations are also bearing walls.
Bearing walls usually have perpendicular joists or rafters crossing or resting on top of them and foundations underneath them. An exception are the end walls of a gable-roofed house; these usually run parallel to rafters and joists but must bear the weight of extensive wall framing.
Wind and seismic loads, which produce lateral stresses on a house, are managed by tightly interlocking framing members.
Plywood sheathing or wood or metal crossbracing interconnect framing members, creating a sturdy triangular form and-together with foundation bolts-lock walls to foundation.
