This is a ridiculous concept, which mitigates the usefulness of the Limit Order Book, on the NYSE. Specialists capriciously use this "rule" to penny in front of limit orders, at convenient times. People willing to pay market get price improved (but they were willing to pay at market in the first place !), whereas people with limit orders at the top of the book, get screwed ! The new NYSE regime needs to look closely at this disrupter of normal market supply/demand activity, and make some alterations. This was a Grasso-special, and needs to be removed just like Grasso was.