The development of the concept of seriation was studied for 415 children, ranging in age from 3 to 9 years. The subjects were required to learn to identify the larger or smaller object in a two stimulus series, the smallest or middle-sized object in a three stimulus series, and the largest or next to the smallest in a four stimulus series. The end positions of the series were significantly easier to identify than the inner positions. The four stimulus series was significantly more difficult to learn than a three stimulus series only in the cases in which the subject was required to identify an inner position, but not when he was required to recognize an end position. (Author)