Combining price streams from several providers into a single book, so the broker shows the best bid and best offer available across all of them.
An aggregator takes, say, ten streams and builds a synthetic book. The displayed spread can be tighter than any single provider's, because the best bid may come from one firm and the best offer from another.
The risk is depth. An aggregated top of book may hold only small size, so a large order sweeps through several levels and fills worse than shown. This is a common source of slippage that traders mistake for broker misconduct.
Example: aggregated EUR/USD shows 1.08401 / 1.08413 in 1,000,000. An order for 5,000,000 fills 1,000,000 at 1.08413, 2,000,000 at 1.08418 and 2,000,000 at 1.08425, an average of 1.08420, 0.7 pipettes worse than the screen.
Original diagrams for the ideas on this page. Illustrative, not real market data.
Slippage on a market order. You click at 20.00, but only 300 shares are resting there, so the rest of the order fills at 20.01, 20.03 and 20.04. The average price paid is 20.02, and that two-cent gap is slippage.The bid-ask spread. Buy orders sit below, sell orders above, and the gap between the best bid (50.01) and best ask (50.04) is the spread you pay to cross. Bar length shows the size resting at each price.
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