This paper examines pairwise assortment similarities at U.S. supermarkets to understand how assortment composition and size are related to underlying factors that describe local store clientele, local competitive structure, and the retail outlets' characteristics. The top-selling items, which cumulatively make up 50% of sales, are sold at nearly every store, but other items are viewed as optional. We find that, within states, supermarkets owned by the same chain carry similar assortments and that the composition of their clientele and the presence of competing stores have effects on assortment similarity that are an order of magnitude smaller than ownership structure. In contrast, we find that, across states, supermarkets owned by the same chain do version their assortment. We explain this difference using extant work on the minimal efficient scale of supermarkets and on local demand effects. Furthermore, we investigate the distribution and role of regional brands. We find that regional brands are primarily distributed by small regional chains or independent stores. “Value” regional brands are primarily distributed by supermarket firms without store brands, whereas the distribution of “premium” regional brands is unrelated to the presence of store brands. We discuss our findings in the context of modeling assortment decisions and manufacturers designing distribution policies.