Inflated therapy balls (gym balls) are simple curvilinear forms (Figure 3.17). Gym balls have neither front nor side, unless implied by surface treatment, seams, or branding logos. They offer a practical, inexpensive, and healthful means of sitting.
Compound curves are curvilinear shapes that bend in two or more directions. Poul Kjsrholm’s PK 9 is composed of a compound curved fiberglass seat covered with leather, with curved steel supports, as seen in the overlay drawings in Figure 3.18. The chair’s profile created an exceptional challenge regarding its compound curve in order to apply leather to the chair’s form. The solution came about by making a fiberglass mold and applying the leather over the mold after getting the leather wet and stretch-fitted over a wooden model. The leather then was applied to the fiberglass mold loosely to react to the shifting weight of the user. The side profile was edged with a modest border on the chair’s backside. Designed between 1959 and 1961, the chair has been fabricated by Fritz Hansen since 1982 (Figure 3.19).
Cesare Leonardi and Franca Stagi designed the Dondolo rocker in 1967. It is made from molded fiberglass-reinforced polyester strengthened by incorporating ribbing, and has an unusual curvilinear shape, which relies on the posture and weight of the user to function properly (Figure 3.20).