would be for Bic to sell its pens only with Bic lighters. (However, a company may offer a limited free item with another purchase as a promotion.) Vertical tying is the practice of requiring customers to purchase related products or services together, from the same company. For example, a company might mandate that its automobiles could only be serviced by its own dealers. In an effort to curb this, many jurisdictions require that warranties not be voided by outside servicing; for example, see the Magnuson-Moss Warranty Act in the United States. In United States law Certain tying arrangements are illegal in the United States under both the Sherman Antitrust Act, and Section 3 of the Clayton Act. A tying arrangement is defined as "an agreement by a party to sell one product but only on the condition that the buyer also purchases a different (or tied) product, or at least agrees he will not purchase the product from any other supplier." Tying may be the action of several companies as well as the work of just one firm. Success on a tying claim typically requires proof of four elements: (1) two separate products or services are involved; (2) the purchase of the tying product is conditioned on the additional purchase of the tied product; (3) the seller has sufficient market power in the market for the tying product; (4) a not insubstantial amount of interstate commerce in the tied product market is affected. For at least three decades, the Supreme Court defined the required "economic power" to include just about any departure from perfect competition, going so far as to hold that possession of a copyright or even the existence of a tie itself gave rise to a presumption of economic power. The Supreme Court has since held that a plaintiff must establish the sort of market power necessary for other antitrust violations in order to prove sufficient "economic power" necessary to establish a per se tie. More recently, the Court has eliminated any presumption of market power based solely on the fact that the tying product is patented or copyrighted. In recent years, changing business practices surrounding new technologies have put the legality of tying arrangements to the test. Although the Supreme Court still considers some tying arrangements as per se illegal, the Court actually uses a rule-of-reason analysis, requiring an analysis of foreclosure effects and an affirmative defense