In errorless discrimination, the trainer does not allow the organism to make mistakes by responding to the extinction stimulus. Initially SD and SI are very different, but differences between the stimuli are gradually reduced as training progresses. The procedure eliminates the aversive effects of extinction that are generated by other discrimination-training methods. For example, pigeons flap their wings in an aggressive manner and will work for an opportunity to attack another bird during the presentation of the SI on a multiple schedule. This behaviour does not occur when errorless discrimination is used in training.