A more thorough explanation:

An agreement between a seller and a buyer where the seller makes the sale of one product (the "tying" product) conditional on the buyer's agreement to purchase a separate product (the "tied" product) from the seller. Alternatively, it is also considered a tying arrangement when the seller conditions the sale of the tying product on the buyer's agreement not to purchase the tied product from any other seller.

For example, a printer manufacturer may require customers to purchase their ink cartridges in order to use their printers. This is a tying arrangement because the sale of the printer is tied to the sale of the ink cartridges.

Tying arrangements are not necessarily illegal, but they can be if they are used to maintain or increase the seller's market power or limit competition in the market for the tied product. If a tying arrangement is illegal, it may be considered illegal per se or illegal under the rule of reason.

An example of an illegal per se tying arrangement would be if a cable company required customers to purchase their cable box in order to access their cable service. This is illegal because the cable company has enough market power to limit competition in the market for cable boxes.

An example of an illegal tying arrangement under the rule of reason would be if a software company required customers to use their web browser in order to access their software. This may be considered an unreasonable restraint on trade because it limits competition in the market for web browsers.