Spatial language and cognition vary across contexts. In some groups, people tend to use egocentric space (e.g. left, right) to encode the locations of objects, while in other groups, people use allocentric space (e.g. upriver, downriver) to describe the same spatial scene. These different spatial Frames of Reference (FoRs) characterize both the way people talk about spatial relations and the way they think about them, even when they are not using language. These patterns of spatial language and spatial thinking tend to covary, but the root causes of this variation are unclear. Here we propose that this variation in FoR use reflects variation in the spatial discriminability of the relevant spatial continua. In an initial test of this proposal, we compared FoR use across spatial axes that are known to differ in discriminability. In two non-verbal tests, a group of indigenous Bolivians used different FoRs on different spatial axes; on the lateral axis, where egocentric (left-right) discrimination is difficult, their behavior was predominantly allocentric; on the sagittal axis, where egocentric (front-back) discrimination is relatively easy, their behavior was predominantly egocentric. These findings support the spatial discriminability hypothesis, which may explain variation in spatial concepts not only across axes, but also across groups, between individuals, and over development.