Human and non-human species possess a mental system of number representations that appear early in the lifespan and that supports approximate number skills, such as numerical estimation or number comparison. With the later acquisition of language and of symbolic numbers, human beings also develop exact number skills that allow using numbers precisely, such as in counting and arithmetic. The current review points out the behavioral data which either support or challenge these contrasting proposals. In an attempt to provide a comprehensive overview of these mixed data, we carefully took into account the heterogeneity of the available studies regarding the kind of tasks, stimuli or ages of assessment. Also, you can show how the numbers are related simply by identifying the tenth place value of each digit.
