Abstract: A cushion for the seat of a chair, has at least two groups of inflatable tubes arranged in a side-by-side relationship and oriented in a direction corresponding to the front to back direction of the chair seat. The tubes of each group are inflatable together. The first group is a single tube or two or more adjacent tubes, and the other group is at least two spaced apart tubes arranged on opposite sides of the first group of tubes. The groups of tubes are inflatable sequentially. Symmetrical but periodically varying support for the user is thus provided.