Many retailers use seemingly innocuous dividing lines to separate product alternatives on their websites or product catalogs. Although previous research on advertising design has focused on the impact of dividing lines on symbolic meanings or perceived categorization, the present research argues that, beyond symbolic or categorical functions, a dividing line can influence consumers' perceived quantity of the product alternatives displayed. Across 10 studies (including an eye-tracking study and three additional studies reported in the Supporting Information Appendix), our results show that consumers perceive a smaller number of products displayed on a page when these products are separated by a dividing line compared to when they are not. This effect occurs because the dividing line separates the products into top versus bottom (or left vs. right) segments, such that participants' visual attention is largely drawn to the top (or the left) where their eyes first fixate. Consequently, participants tend to estimate the total number of items based on the subset they pay attention to. In addition, the effect is attenuated when participants' attention is directly drawn to the segment they have previously neglected and can hold regardless of line orientation. Finally, it can have several marketing outcomes, such as higher willingness to buy and lower post-choice satisfaction.