Sub-Penny Pricing
Sub-penny pricing refers to the practice of executing or quoting equity trades at price increments smaller than one cent — such as $0.001 or fractions thereof — which is prohibited for displayed quotes on U.S. exchanges under SEC Rule 612 but is permitted in certain off-exchange execution contexts, creating an asymmetry that market makers use to gain queue priority without publicly improving displayed prices.
SEC Rule 612, adopted in 2005 as part of Regulation NMS, prohibits market participants from displaying, ranking, or accepting bids or offers for NMS stocks at a price increment smaller than one cent, with limited exceptions for stocks priced below one dollar. The rule was intended to prevent a fragmentation of the order book into thousands of microscopic price levels that would have been difficult for investors and market centers to manage.
However, Rule 612 applies specifically to displayed quotes and exchange order books. It does not prohibit off-exchange execution at sub-penny prices. This distinction has created a structural asymmetry: wholesale market makers executing retail orders through payment for order flow arrangements routinely fill customer orders at prices fractionally better than the national best bid or offer — for example, at $50.0025 rather than the displayed $50.00 — providing price improvement of a fraction of a cent that technically satisfies best execution standards while still allowing the market maker to capture most of the spread.
Critics argue that sub-penny executions off-exchange effectively allow wholesalers to jump the queue of displayed limit orders without improving upon their prices in any economically meaningful way. A displayed limit order resting at $50.00 seeking to buy shares provides genuine price improvement over the $50.01 offer and should, in principle, be entitled to fill priority. When a wholesaler fills a marketable retail order at $50.0025 — price-improving by one-quarter of a cent — the retail customer receives a marginally better price but the displayed limit order provider receives nothing, even though they posted the liquidity that defined the competitive spread.
The sub-penny debate has been a central element of broader discussions about payment for order flow and market structure reform. In its 2022 equity market structure proposals, the SEC proposed updating Rule 612 to allow displayed quotes at sub-penny increments for certain securities, effectively leveling the playing field between displayed and off-exchange liquidity providers. The proposal argued that modernizing tick size rules could improve competition and execution quality by enabling limit order providers to compete on equal footing with internalizing wholesalers.