Rule proposed by Percival to establish whether a patient is going to experience discomfort in binocular vision. It states that if Donders' line (or demand line) lies within the zone of comfort, which is the middle third of the total range of relative convergence (to the blur points), Percival's criterion of comfortable binocular vision is fulfilled. If it is not, appropriate prisms, spherical lenses or visual training can be used to shift the demand point within the zone of comfort. In this criterion, no reference is made to the actual phoria of the subject and for this reason it has been criticized by several authors. Syn. middle third technique. See relative convergence; binocular vision single zone of clear.