Convertible seat for automobile vehicles

A seat structure comprising a seat proper, means for mounting a front part of the seat proper on the floor of a vehicle to pivot about a first transverse axis a backrest having a base part, lateral members mounting the base part of the backrest on the seat proper to pivot about a second transverse axis, at least one link which is connected adjacent a first end of the link to the base part of the backrest to pivot about a third transverse axis, means for connecting the link adjacent a second end of the link opposite said first end to the floor to pivot about a fourth transverse axis, the seat proper and the link forming two crossed arms of a device for deploying in the horizontal position the backrest relative to the seat proper upon a rotation of the seat proper through an angle of about 180.degree. in a direction away from the backrest about said first axis, said third transverse axis being embodied by pins which are engaged in slots provided in said lateral members and hook members being respectively pivoted to the corresponding lateral members and being respectively engageable with the pins to hold the pins at one end of the slots, said hook members being part of a device for selectively immobilizing and releasing the pins relative to the slots.