An order that follows, and then is placed at, the best bid, when buying a security (a buy order), and the best offer, when selling (a sell order). Primary peg orders are instrumental for traders who seek to reach at the best possible price as the price of a security fluctuates within a certain range.

Such an order pegs to the passive side of the national best bid and offer (NBBO). It can be placed as either displayed (displayed order) or non-displayed (non-displayed order) in an exchange’s trading Book. An exchange’s members may have the option of combining a primary peg order with a limit order: an order at which price the primary peg order will remain in the trading book if the best bid and offer diverts from the limit price in either direction.