The paper builds an analytically tractable model that illustrates the "proximity-concentration trade-off" involved in horizontal multinationals. For low trade costs, firms are single-plant firms, for intermediate costs, some are single-plant firms whereas others are multinationals, for large trade costs, firms are multinationals. Because of the modeling strategy, the model is suited for a welfare analysis of multinationals. It shows that too many firms choose to concentrate their production in only one location. Also, for some transport costs, a reduction in transport costs worsens welfare.