Thanks, as ever, Maeve. But I don't think that is the one I ref to, which is not used by the chair umpire himself, but by someone sitting on a chair behind and sightly to one side of his; who follows the play with it, swivelling side to side as the ball travels and stopping between the points; and also leaves the chair to point it at the toss-up as it occurs at the beginning of each match.

~M~