A busk is a piece of stiff material, usually with less spring or give than the material used to reinforce the rest of a corset . It has changed its appearance over time, but it is still an integral part of functional corsets today.
Busks are not as old as corsets themselves, but begin to appear in the 16th century with the development of the pair of bodies as a separate garment from the dress bodice. The primary goal was likely to aid in forming the appropriate silhouette of a lady's torso by creating a straight front panel to guide the eye, but secondarily it also aided in producing the fashionably correct posture by making it very uncomfortable for the wearer to slouch at all.
Initially, busks were likely formed out of extra-thick bundles of broomstraw or bents used in the rest of the stays before developing into the flat wooden or whalebone paddles encased in the front pocket of the garment. The more durable materials allowed busks to be reused for many different bodies over the course of the wearer's life, and so they were often decorated with painting or carvings, sometimes by a lover, so that the images might be kept close to the heart.
The use of this type of busk necessitates a back-closing corset, however, and so was not used in every pair of stays, especially those worn by women in working classes who could not afford a ladies' maid or assistance in dressing themselves. Front-lacing stays had edges reinforced with the smaller bundles of reeds or pieces of whalebone used in the rest of the garment.
Single-piece busks fell out of fashion in the 19th century when silhouettes changed once more. First, the busk was entirely absent from the design of early Regency-era stays, which were quite short and chiefly meant as bust support. Second, the advent of steel and its use in corsetry made front-closing busks easy to use and very cheap to manufacture, and the old-fashioned busk disappeared from use altogether.