A seat comprising a seat portion and a backrest portion, and in which the seat portion is one wherein: (i) the seat portion has a concave recess for receiving a person's buttocks; (ii) the seat portion is made of a hard material which is such that the seat portion does not compress when the seat portion is sat upon by a person; (iii) the concave recess extends into a first concave passage portion for receiving a rear thigh part of a first leg of the person, and into a second concave passage portion for receiving a rear thigh part of a second leg of the person; (iv) the second concave passage portion is spaced apart from the first concave passage portion by a raised seat portion extending to the periphery of a front edge of the seat portion; (v) the first and second concave passage portions (12, 14) extend to the periphery of the front edge of the seat portion; (vi) the concave recess is for receiving a person's buttocks and extends transversely of the seat portion towards side edges of the seat portion; and (vii) the distance of the concave recess for receiving a person's buttocks, from the side edges of the seat portion Is less than the distance of the first and second concave portions (12, 14) from the side edges of the seat portion; and the backrest portion is one wherein: (viii) the backrest portion is made of a hard material which does not compress when the backrest portion is lent on by the person; (ix) the backrest portion has a concave recess for receiving the person's spine; (x) the concave recess for receiving the person's spine extends into a transversely extending passage portion which is for receiving the person's shoulders and which extends towards side edges of the backrest portion; and (xi) the distance of the transversely extending passage portion for receiving the person's shoulders from the side edges of the backrest portion is less than the distance of the concave recess for receiving the person's spine from the side edges of the backrest portion. (i) the seat portion has a concave recess for receiving a person's buttocks; (ii) the seat portion is made of a hard material which is such that the seat portion does not compress when the seat portion is sat upon by a person; (iii) the concave recess extends into a first concave passage portion for receiving a rear thigh part of a first leg of the person, and into a second concave passage portion for receiving a rear thigh part of a second leg of the person; (iv) the second concave passage portion is spaced apart from the first concave passage portion by a raised seat portion extending to the periphery of a front edge of the seat portion; (v) the first and second concave passage portions (12, 14) extend to the periphery of the front edge of the seat portion; (vi) the concave recess is for receiving a person's buttocks and extends transversely of the seat portion towards side edges of the seat portion; and (vii) the distance of the concave recess for receiving a person's buttocks, from the side edges of the seat portion Is less than the distance of the first and second concave portions (12, 14) from the side edges of the seat portion; and the backrest portion is one wherein: (viii) the backrest portion is made of a hard material which does not compress when the backrest portion is lent on by the person; (ix) the backrest portion has a concave recess for receiving the person's spine; (x) the concave recess for receiving the person's spine extends into a transversely extending passage portion which is for receiving the person's shoulders and which extends towards side edges of the backrest portion; and (xi) the distance of the transversely extending passage portion for receiving the person's shoulders from the side edges of the backrest portion is less than the distance of the concave recess for receiving the person's spine from the side edges of the backrest portion. (viii) the backrest portion is made of a hard material which does not compress when the backrest portion is lent on by the person; (ix) the backrest portion has a concave recess for receiving the person's spine; (x) the concave recess for receiving the person's spine extends into a transversely extending passage portion which is for receiving the person's shoulders and which extends towards side edges of the backrest portion; and (xi) the distance of the transversely extending passage portion for receiving the person's shoulders from the side edges of the backrest portion is less than the distance of the concave recess for receiving the person's spine from the side edges of the backrest portion.