The rules above are only valid for a perfect competition market: In such a market, there are many buyers and sellers, and no single buyer or seller can directly influence the price. If either the buyers or the sellers can influence this price, the market will react differently. Sometimes there is only one supplier (a monopoly). When this happens, the supplier chooses the price and the demand only determines how much is bought. The supplier will probably raise the price very high as long as people are still willing to buy at that price. If there is only one buyer, the market is called monopsony, and that buyer is able to fix the price.