A wheelchair sit-system with a three dimensional anatomical shape, which can accomplish an equal as well as an unequal pressure/load distribution, stabilize the pelvic base and trunk, compensate sit pathology, and be individually adapted on the basis of treatment of specific sit complaints/impairments. The wheelchair sit system (i.e., a sit cushion and a backrest) is developed methodically via inventory of the three dimensional pelvis/trunk measures; design a three dimensional drawing of basic sit cushions and backrest models in three sizes (S, M and L); specification of the sit pathology of the pelvis and trunk; and modification of the designed three dimensional basic sit cushion and backrest models on the basis of sit pathology. A product development trajectory results in nine sit cushion models in three sizes (S, M and L); six backrest models in three sizes (S, M and L); one generic sit cushion, with inserts, in three sizes (S, M and L); and one generic backrest, with inserts, in three sizes (S, M and L). The assortment of sit cushions and backrest models is applied via a prescription procedure has been developed.