Two experiments tested the ability of 4- and 8-year-old children to encode the extent of a target dowel and later discriminate between the target and a foil having a novel extent. By manipulating the heights of containers in which we presented the stimuli we tested whether children used the relation between the dowels and containers for encoding extent. We found that 8-year-olds encoded extent without relying on the relation between the target dowel and container but 4-year-olds only encoded the extent of the target dowel relative to the container. This early ability to encode extent relative to an aligned standard may serve as a perceptual basis for the developing ability to measure.