Articulated double back for chairs

A chair back comprising at least one back-supporting frame member, a lower back, means mounting the lower back on the frame member for backward tilting movement about a first horizontal, transverse axis and for yieldably restraining such backward tilting, the first axis being located substantially above the seat at a location corresponding to about the middle of the anatomical back of an adult human sitting in the chair, an upper back, and means mounting the upper back on the lower back for backward tilting movement of the upper back relative to and independently of the lower back about a second horizontal, transverse axis located substantially above the first horizontal axis and for yieldably restraining such backward tilting, the lower back being vertically and transversely co-extensive with the major portion of the middle part of the anatomical back of an adult human, and the upper back being verticallly and transversely co-extensive with the major portion of the upper part of the anatomical back of an adult human in the region where the spine curves forward when such human is sitting upright and straightens when such human arches backward.