A seat comprising: a base frame; a seat base that is supported by the base frame; a backrest comprising a backrest support pivotally attached to the base frame at a rear of the seat base to tilt forward and backward relative to the seat base; and a backrest angle adjustment system, characterized in that the backrest angle adjustment system comprises: an angle adjustment slot located substantially centrally on the backrest support and extending substantially vertically along the backrest support; an angle adjuster comprising a front face and rear face and further comprising a first bore extending between the front face and the rear face, wherein the angle adjuster is located at the rear of the backrest support and is configured to slide along the angle adjustment slot; a linkage pivotally attached to the base frame and to the angle adjuster; and a locking pin that projects through the angle adjustment slot and through the first bore of the angle adjuster and that comprises a threaded end that engages with a threaded fastener at the rear face of the angle adjuster, wherein the fastener may be loosened by rotating the fastener in a first direction to allow the locking pin to slide along the angle adjustment slot to cause the backrest support to tilt relative to the seat base, and wherein the fastener may be tightened by rotating the fastener in a second direction to move toward and clamp against the angle adjuster to secure the locking pin in position within the angle adjustment slot to lock the angle of tilt of the backrest support relative to the seat base. a seat base that is supported by the base frame; a backrest comprising a backrest support pivotally attached to the base frame at a rear of the seat base to tilt forward and backward relative to the seat base; and a backrest angle adjustment system, characterized in that the backrest angle adjustment system comprises: an angle adjustment slot located substantially centrally on the backrest support and extending substantially vertically along the backrest support; an angle adjuster comprising a front face and rear face and further comprising a first bore extending between the front face and the rear face, wherein the angle adjuster is located at the rear of the backrest support and is configured to slide along the angle adjustment slot; a linkage pivotally attached to the base frame and to the angle adjuster; and a locking pin that projects through the angle adjustment slot and through the first bore of the angle adjuster and that comprises a threaded end that engages with a threaded fastener at the rear face of the angle adjuster, wherein the fastener may be loosened by rotating the fastener in a first direction to allow the locking pin to slide along the angle adjustment slot to cause the backrest support to tilt relative to the seat base, and wherein the fastener may be tightened by rotating the fastener in a second direction to move toward and clamp against the angle adjuster to secure the locking pin in position within the angle adjustment slot to lock the angle of tilt of the backrest support relative to the seat base. an angle adjustment slot located substantially centrally on the backrest support and extending substantially vertically along the backrest support; an angle adjuster comprising a front face and rear face and further comprising a first bore extending between the front face and the rear face, wherein the angle adjuster is located at the rear of the backrest support and is configured to slide along the angle adjustment slot; a linkage pivotally attached to the base frame and to the angle adjuster; and a locking pin that projects through the angle adjustment slot and through the first bore of the angle adjuster and that comprises a threaded end that engages with a threaded fastener at the rear face of the angle adjuster, wherein the fastener may be loosened by rotating the fastener in a first direction to allow the locking pin to slide along the angle adjustment slot to cause the backrest support to tilt relative to the seat base, and wherein the fastener may be tightened by rotating the fastener in a second direction to move toward and clamp against the angle adjuster to secure the locking pin in position within the angle adjustment slot to lock the angle of tilt of the backrest support relative to the seat base.