Primary and secondary stability: Although every kayak will rock from side-to-side, wider kayaks with more buoyancy away from the centerline will present more resistance to tipping and thus feel less likely to capsize than a narrow one with less buoyancy away from the centerline. Flat-bottomed boats that push their volume away from the centerline will also feel more stable than rounded or V-shaped hull shapes that distribute buoyancy more evenly.
While flat-bottomed boats have more primary (sometimes called "initial") stability, and feel more stable to the beginner they usually have less secondary stability. Once they do begin to tip, they capsize quickly and suddenly. Rounder-bottomed boats are quite the opposite having lower initial or primary stability and (usually) greater secondary stability. The chine in some boats increases secondary stability by effectively widening the beam of the boat when it is heeled (tipped).
Secondary stability refers to final stability, or additional resistance to capsizing as a kayak approaches capsizing. Rounder-bottomed boats present a greater cross-section to the water as they are tipped from level ("heeled"), while very flat-bottomed boats present less. Sea kayaks, designed for open water and rough conditions, are generally narrower (22-25 inches) and have more secondary stability than recreational kayaks, which are wider (26-30+ inches), have a flatter hull shape, and more primary stability. Kayaks with only moderate primary, but excellent secondary are, in general, considered more seaworthy, especially in challenging conditions.