A stool is a kind of chair with no back or arm rests but with only a bottom to sit on. Said bottom is usually smaller than a chair, ranging from smaller than ones tushy to moderately commodious models.
Modern stools are often circular with four or sometimes three legs, however, medieval models can vary considerably from this, being square, or triangular on occasion.
A popular model of medieval stool was the "folding stool" or "camp stool", a hinged stool which could be folded up flat and easily and transported. Most were inspired in design by Roman senator's models and some non-portable stools retained this form for status reasons, despite loosing the portability. The basic form viewed side on is:
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Other stool designs are occasionally also observed in the medieval record, such as a three-legged triangular stool.
Stools and benches were probably more common in medieval life than chairs - only higher ranking persons would merit a chair with a backrest until late period. This can be difficult for the modern person who has not developed the back muscles to relax while holding themselves upright.