The chair has a seat that folds down about a horizontal axle 24 and which is intended, in particular, to equip halls that receive the public, such as lecture theatres and/or show halls. This chair comprises at least an underframe having an essentially longitudinal axis 14 and to which a backrest 16 may be fixed, and with respect to which a fixed horizontal axle 24 is determined. The seat 20 comprises a first part 32 projecting forwards with respect to the horizontal axle 24, and a second part 30 projecting towards the underframe 12. The underframe 12 contains a gas strut 40, one end 48 of which is fixed to the underframe 12, and the other end 50 of which is fixed to the second part 30 of the seat 20.