And as a refresher for why HFT exists - it's a side effect of RegNMS. RegNMS exists because the guys in funny jackets in NY with palm pilots were ripping people off.
There was no consolidated tape and obligation for exchanges to route your order for Best Execution. There was no National Best Bid Best Offer.
There was just whatever price the exchange your broker sent your order to filled you at.
Some exchanges cough NASDAQ cough used to do things like display sub-penny quotes even though they only filled at full penny increments. So they could attract flow advertising prices they wouldn't file you at. See Rule 612.
There was no consolidated tape and obligation for exchanges to route your order for Best Execution. There was no National Best Bid Best Offer.
There was just whatever price the exchange your broker sent your order to filled you at.
Some exchanges cough NASDAQ cough used to do things like display sub-penny quotes even though they only filled at full penny increments. So they could attract flow advertising prices they wouldn't file you at. See Rule 612.