British Standard definitions for chairs
BS 7973 defines the various types of chairs available as follows.
chair - componant in sizes of 50mm to 300 mm normally manufactured from steel wire conforming to BS 4482 commonly used to support top top horizontal reinforcement or to separate vertical reinforcement in walls
individual chairs - chair manufactured in standard heights of between 75 mm and 300 mm inclusive and used to support reinforcement at one point.
continuous chair - chair manufactured to privide a straight line support at a uniform height normally in the range of 45 to 300 mm, normally available in lengths up to 2 m.
circular chairs - steel chair delivered flat and formed into circles on site.