Abstract: A chair back supports a positionable headrest assembly including a post and headrest with the post manually positionable in a channel on the chair back. A retainer arm bears against the post to inhibit post travel against all but deliberately applied force, as for example, by a dental assistant when adjusting a dental chair to accommodate the next patient. A threaded member in the chair back is settable to apply the desired resistance to post travel during headrest adjustment. Access to the threaded member for adjustment purposes may be by an opening in the chair back normally receiving a fastener securing a chair component.