School chairs present several particular design difficulties. The need for low cost and durability dictate the use of a firm, non-adjustable chair. The design of a chair to provide the best fit for a population is not simply a matter of fitting the average size person from the population. The appropriate dimensions of a chair to provide the best fit for a population can be determined by ensuring that 'the smallest can fit and the largest can reach.' Just as the anthropometric dimension of an average size person is not necessarily the appropriate dimension to use for determining a dimension of a chair for that population, so too the angles and contours suitable for an average size person are not necessarily the most suitable for the population as a whole. Unfortunately clear principles that enable fitting the dimensions of a chair to a population do not appear to exist for fitting the angles and contours of a chair to a population, particularly a school population. An understanding of the mechanics of sitting is a prerequisite to addressing the question of fitting a chair to a population. Chapter 2 presents a qualitative discussion of the mechanics of the interaction between a chair and its occupant. In addition to the commonly considered chair design features such as the heights and angles of the seat and backrest the contour of the seat is also shown to be an important element in the overall functioning of a chair. The purpose of the experiments described in Chapter 3 are to determine the appropriate seat position for the largest size school chair in Australian schools. An experimental chair was constructed with the height of the front of the seat, seat depth, and seat width as described by Sebel Furniture (1995) and Australian Standards (1995). The experiments in Chapter 3 consisted of two groups of 16 Year 11 students adjusting the rear seat height (seat angle) on an experimental chair to their preferred height. The first group adjusted the rear seat height during a simulated classroom activity while sitting at a desk as used in their usual classroom. As the results from this group suggested that the desk height could have influenced the preferred rear seat height a second group adjusted the rear seat height for a brief adjustment period while sitting at three different desk heights. The results indicate that there were no significant differences between the long and short adjustment periods and that the seat angle corresponding to the preferred rear seat height was between -1.1 to + 2.7 degrees. The preferred rear seat height of students exhibited a negative correlation with their popliteal height and a positive correlation with the height of the desk being used at the time. In other words students with a higher popliteal height preferred a lower rear seat height and when a higher desk was used, the students preferred a higher rear seat height. In Chapter 4 a second experimental chair with a purpose built contour measuring device (bumograph) mounted in place of the seat was used to measure seated buttock contours. The position of the bumograph corresponded with the seat position determined in Chapter 3. The buttock contours of 16 Year 11 students were measured while sitting on the experimental chair in a range of postures used in the school setting. One anterior-posterior (AP) and one lateral profile were extracted from the measured contours and six dimensions from these profiles were analysed for common features and systematic variations related to posture, mass, or gender which may assist in the design of future school chair seats. The data showed consistent patterns in the general shape of both the AP and lateral profiles. Five out of the six profile dimensions were significantly different for males and females. In contrast only one dimension for one pair of postures demonstrated significant differences. A synthesis of the theoretical and experimental work from Chapters 2-4 is presented in Chapter 5. Particular attention is given to how the findings of the present studies may be applied to future research and to school chair design.