The difference between a horizontal merger and a vertical merger is that Multiple Choice companies operating at different but related levels of an industry merge in a horizontal merger, whereas two firms in unrelated industries merge in a vertical merger. firms that make and sell similar products to the same customers merge in a horizontal merger, whereas companies operating at different but related levels of an industry merge in a vertical merger. companies operating at different but related levels of an industry merge in a horizontal merger, whereas firms that make and sell similar products to the same customers merge in a vertical merger. O two firms in unrelated industries merge in a horizontal merger, whereas firms that make and sell similar products to the same customers merge in a vertical merger. one corporation merges with one of its customers or suppliers in a horizontal merger, whereas a firm merges with its direct competitor in a vertical merger