The representation of numbers is commonly viewed as an ordered continuum of magnitudes, referred to as the "mental number line." Previous work has repeatedly shown that number representations evoked by a given task can be easily altered, yielding an ongoing discussion about the basic properties of the mental number line and how malleable they are. Here we studied whether the resolution of the mental number line is fixed or depends on the relative magnitudes that are being processed. In 2 experiments, participants compared the same number pairs under 2 conditions that differed in terms of the overall range of numbers present. We report a novel number range effect, such that comparisons of the same number pairs were responded to faster under the smaller versus larger number range. This finding is consistent with the idea that the resolution of the mental number line can be adjusted, as if a unit difference is perceived as larger in smaller ranges.