Young children typically show strong biases when estimating theplacement of numbers on or along a scale. Number line estimationchanges in accuracy and linearity across development. However,existing research is almost entirely based on a horizontal numberline, which presupposes that numbers are scaled on a horizontalplane only. We present data that broaden our understanding ofnumber line estimation by also including vertically oriented scales.This study presented 4- to 7-year-olds with the number line estimationtask presented in both horizontal and vertical orientationsand on different scales. Our results suggest that children storenumbers as accurately in the vertical plane as in the horizontalplane, although some developmental changes are observed. Ourresults highlight how even simple experimental manipulationscan reveal the complexities of internal representations of number.