Below about 30% moisture content — the fiber saturation point — wood shrinks as it dries and swells as it takes on moisture. Above that point, extra water just fills cell cavities and dimensions hold still. Everything about seasonal movement happens between roughly 6% and 14% MC indoors, and it happens almost entirely across the grain: length changes are a rounding error (0.1–0.2% green to dry, total).
Wood shrinks about twice as much around the growth rings (tangential) as across them (radial). A flatsawn board's width is mostly tangential movement; a quartersawn board's width is mostly radial — which is the whole secret of quartersawn stability: same species, roughly half the width change. The USDA publishes both numbers per species; our movement calculator interpolates between moisture contents with the Handbook's own formula instead of the "1/4 inch per foot" folklore that overstates stable species and understates lively ones.
Tabletops must be attached so they can slide: slotted screw holes, buttons, or figure-8 fasteners, with the expected movement (from the calculator) as the slot length. Frame-and-panel doors and lids need panel room: groove depth minus panel tongue should exceed the panel's growth, and panels are finished before assembly so a shrink line never shows bare wood. Drawers stick in summer because the box sides and the opening both move — side clearance is a seasonal decision, and a drawer fitted piston-tight in January is honest work; the same fit in August is a callback. Breadboard ends only work glued at the center, pinned in slots toward the edges.
Shrinkage varies board to board — the Handbook itself notes a coefficient of variation around 15% on tangential values. Treat any movement figure as the center of a range, and build in allowance 25% beyond it. The math gets you to the right joint design; it cannot certify a particular board.