Stretcher

A stretcher is a horizontal member connecting two or more legs of a chair, table, or similar furniture piece, positioned below the seat or tabletop -- its job is almost entirely resisting racking force, keeping the legs from splaying or the whole structure from racking sideways under lateral load. Stretchers let legs be made noticeably slimmer and more visually light than they'd need to be to resist the same racking forces entirely on their own at the top joint, since the stretcher shares that structural load lower down.

Box stretchers (four stretchers forming a rectangle low between the legs) and H-stretchers (two long stretchers connected by one shorter cross-stretcher) are the two most common configurations, chosen as much for the piece's overall visual proportions as for the marginal structural difference between them.