A stool is a form of chair without back or armrests. Usually a stool consists simply of a seat mounted on legs -- typically three or four legs, but modern stools come also in swivel chair style. The seat may be round, or square, or of some other shape.
A "high" variant of a stool is a bar stool, which sometimes comes with a short back.
A schoolroom "corner stool" with a dunce cap.
Stools in spanking
For the spanker
A very low stool (20 cm) can be useful for spanking a heavy person such as an adult, as this allows the spankee to rest some of their weight on their hands and knees (a crossover between the OTK and the on-all-fours position). Such a low stool means however that the spanker's legs must be stretched forward.
For the spankee, to sit on
In school corporal punishment it was not uncommon to require a delinquent to sit on a special stool as a form of humiliation. This might also serve as a form of corner time, and was sometimes combined with the wearing of a dunce cap.
Requiring a recently spanked person to sit on a stool can prolong and intensify the sting, particularly if the seat of the stool is unpadded. Because the stool has no back or arms, all the weight of the spankee rests on his or her bottom. A punishment stool is a special type of stool equipped with spikes to make sitting on it an ordeal.
For the spankee, to lie or bend over
In the Melody stories by Lurking Dragon (and other stories using the same setting, Rejuve Universe) a special kind of stool, known as a CornerStool™ was described. It had regular raised areas (of different shapes and sizes for different degrees of severity) to irritate the bare spanked bottom of a spankee.