Limit Order Protection [ Ссылка ]
When a customer places a market order to purchase a stock, it would be effected at the ask price plus a commission or a markup. However, if a customer limit order is accepted, the member firm is now obligated to handle the order essentially as though it were its own in the interdealer market. Therefore if a market maker receives a customer limit order that improves its price (bid or ask) or quote size, it must update its quote immediately. In this case, the market maker must now show 16.50 bid for 1,300 shares. Keep in mind that nothing in the rules requires member firms to accept limit orders from any customer.

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