Also called inside market, NBBO
Top of book
The best bid and best ask and the size resting at each. It is what a market order trades against, and on one venue it is a sample of the whole market, not the whole market.
Updated 2026-09-16
The top of book is the inside market: the best bid, the best ask, and the number of shares resting at each. A market order trades against it, which is why the desk quotes a market buy at the ask and a market sell at the bid, and why the bid-ask spread is the first cost of any trade.
Consolidated across every exchange it is called the NBBO, the national best bid and offer. From a single venue it is that venue's inside only, which can sit a few cents off the national one and can go one-sided after the close. The desk labels its quote IEX for that reason and refuses to price a fill against a one-sided book.
The sizes at the inside are the raw material of the order imbalance read. They are also the smallest, fastest-changing numbers on the screen: a resting 4,000 shares can be pulled in the time it takes to read them.
See also
- Order book — The list of resting buy and sell orders at each price. The inside of it is the bid and the ask; the rest is depth, which the free IEX feed does not publish.
- Bid-ask spread — The gap between the highest price a buyer will pay and the lowest a seller will accept. It is the cost of trading, and you pay half of it on the way in and half on the way out.
- Order imbalance — The share of the displayed size sitting on the bid versus the ask. Bid heavy says buyers are queued; ask heavy says sellers are. A lean, not a forecast, and it changes every second.
- Market order — An instruction to trade immediately at whatever price is available. It guarantees execution and guarantees nothing about the price you get.
Educational content, not investment advice. Angie explains how a valuation is built so you can judge it yourself. What you do with that is your call.
