Equity-market structure in the US has made important advances over the past 20 years, promoting greater transparency and liquidity. Three powerful forces have been at work: technology, regulation and competition. The result has been narrower spreads, faster execution and lower overall explicit costs to trading stocks.

With the overwhelming majority of transactions now done over multiple electronic markets each with its own rule books, the equity-market structure is increasingly fragmented and complex. The risks associated with this fragmentation and complexity are amplified by the dramatic increase in the speed of execution and trading communications.