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It was so complex. The number of destinations for trading stocks was maddening. There were four public exchanges: the NYSE, Nasdaq, Direct Edge, and BATS (the latter two, which specialized in high-speed trading, appeared on the scene in 2005 and 2006, respectively). Inside each of those exchanges were various other destinations. The NYSE had NYSE Arca, NYSE Amex, NYSE Euronext, and NYSE Alternext. Nasdaq had three markets. BATS had two. Direct Edge had EDGA, which had no “maker-taker” system, and EDGX, which did.
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Scott Patterson (Dark Pools: The Rise of the Machine Traders and the Rigging of the U.S. Stock Market)