The mental sequence line is a cognitive construct by which sequences are perceived as beginning on the left and extending rightwards. Its developmental origins are unknown. In the first experiment, 4- and 5-year-old children placed items in order from four ordinal sequences, two of which they may have seen in print (i.e., letters and numbers) and two they would not have (i.e., times of day and meals). Four-year-olds systematically ordered letters from left to right; by age 5, numbers and times of day, but not meals, were ordered this way as well. These data suggest experience that cultural text direction, rather than inherent cortical organization, is responsible for development of the mental sequence line. The second experiment measured the mental sequence line in 6-, 7-, and 8-year-old children, and assessed its relation to visuospatial skills and mathematics ability. Mental sequence lines were measured by Spatial-Numerical Association of Response Codes (SNARC) and distance effects. Children’s mental rotation ability was moderately correlated (r=.303) with the distance effect but neither it nor the SNARC effect correlated with mathematic ability. These data suggest the mental sequence line is not at the core of children’s number sense, as previously hypothesized. The final experiment assessed consistency across time of SNARC and distance effects in adults for numbers, weekdays, and months. Despite often being described as trait variables, SNARC and distance effects were only moderately consistent for numbers, with even more variable results for months and weekdays, results suggesting mental sequence lines are not byproducts of stable, inflexible neural architectures. Combined, my data show sequence-space associations first emerge for letters, and subsequent development supports an enculturation hypothesis. By middle childhood, individual differences are evident in these effects that, as in adults, correlate with measures of spatial cognition. By adulthood, those individual differences have become modestly stable. / Dissertation / Doctor of Philosophy (PhD)