structure to and fro relative to the base 22 by virtue of the swinging movement of swing arms 50 52 relative to the seat and base structures this is illustrated in fig3 and 4 moreover through the use of the swing arms 50 52 the seat structure also is displaced in the horizontal direction relative to the fixed base as the former undergoes rocking motion along a relatively large arc this gives a pleasing gliding sensation reminiscent of that produced by porch gliders in order to allow the seat height of the chair to be designed within a limited range the distance between the upper pivot joints 54 56 and lower pivot joints 58 59 or the distance between the base and seat links 40 41 in the specific embodiment must also be kept within a limited range however it was discovered that such limitations could excessively magnify the leverage of the chair and the occupant 39 s momentum during rocking of the chair to produce an unstable condition in accordance with the invention it was discovered that the ratio of the distance between the upper pivots 54 and 56 and the distance between the lower pivots 58 and 59 must be such as to be capable of producing a certain obtuse triangle among the four bar linkage 40 41 50 and 52 if the chair were rocked to its rear while being vacant of any occupant for conventional chair sizes and styling this triangle which is shown in dotted lines in fig3 must have an angle 34 a 34 of at least six degrees 6 u00b0 but no greater than sixteen degrees 16 u00b0 it has been.