Best Bid and Offer
The Best Bid and Offer (BBO), also known as the National Best Bid and Offer (NBBO) in multi-exchange contexts, represents the highest currently available bid price and the lowest currently available offer price in an order book, defining the inside market and the minimum transaction cost for immediate execution.
In any continuous auction market, the order book at any moment contains a series of limit orders waiting to be filled. On the bid side, participants have submitted orders to purchase at specified prices; on the offer (or ask) side, participants have submitted orders to sell at specified prices. The Best Bid is the highest price any buyer is currently willing to pay; the Best Offer is the lowest price any seller is currently willing to accept. Together, they define the inside market — the top of the order book.
The spread between the BBO — the bid-ask spread — represents the minimum cost of a round-trip transaction (buying and immediately selling). A tight bid-ask spread, common in highly liquid instruments such as Nifty 50 futures or the shares of HDFC Bank, indicates a competitive, efficient market where the cost of immediate execution is low. Wider spreads in illiquid instruments reflect greater risk for market makers and higher transaction costs for participants who need immediate execution.
Displayed versus hidden orders create an important nuance in interpreting the BBO. Most orders visible in the Level 2 order book are 'displayed' orders, meaning their full quantity is visible to all market participants. However, exchanges including NSE also accommodate 'iceberg' orders, where only a portion of the total order size is displayed — the 'Disclosed Quantity' feature. The BBO price levels may be anchored by large hidden orders whose full size is not visible to market participants, leading to a situation where price appears to be near support or resistance even though visible order depth seems thin.
For market orders, the BBO determines the immediate execution price. A market order to buy fills at the current best offer; a market order to sell fills at the current best bid. If the order is larger than the available quantity at the best price, it continues to execute against progressively worse prices — a phenomenon called 'walking the book' — until the order is fully filled. The total cost deviation from the BBO for a large order is the price impact or slippage.
In India's multi-exchange environment, the concept of a National BBO is less formalised than in the United States, partly because NSE dominates equity volume. However, for equity instruments cross-listed on NSE and BSE, smart order routing systems compare the BBO on both exchanges to route orders to the venue offering the better price.