[Rimpinths]

Great point, and you understand the American stock market better than most Americans. This article was true about 20 years ago, but the author is completely wrong when he says this:

"A single market to trade. All stocks for Microsoft (MSFT), are traded on the NASDAQ exchange. All stocks for Ford (F) are on the NYSE."

MSFT and F both traded on 16 difference stock exchanges, not to mention countless "dark pools", each with their own book of bids and offers. But there's a national best bid-offer (NBBO) that all exchanges must respect, so it can behave like a single market. That's what Reg-NMS is about. The benefits of having several exchanges competing each other, while trying to retain the benefits of single market. This is also where HFT enters the picture with latency arbitrage and other trading strategies when prices on those markets get out of sync.

20 years ago, you could say that MSFT only trades on NASDAQ, but that hasn't been true since Reg NMS came into effect in 2005. Each stock has a primary listing market that controls things like halts and opening/closing auctions, but the stock can be traded on any exchange, each with its own dynamics.