Cross-Exchange Sub-Penny Price Discretion Skim
The Formal Definition
The execution practice where wholesale internalizers exploit SEC Rule 612 loopholes across fragmented exchanges, providing a fraction of a sub-penny price improvement (e.g., $0.0002) to satisfy best-execution rules while capturing the remainder of the penny spread on off-exchange ledgers.
Internalizer Retained Spread Skim = Displayed NBBO Spread ($0.01) - Sub-Penny Price Improvement Credit Passed to Client ($0.0002)
Cole Barrett's Reality Check
The Unvarnished Bottom Line"This is the legal loophole that funds the zero-commission brokerage industry. Federal law says public exchanges can't quote stocks in increments smaller than a full penny. But wholesale market makers off-exchange are allowed to trade in sub-pennies. They give you two-hundredths of a cent in 'price improvement' to legally claim they beat the public exchange, and pocket the other 98% of the penny spread for themselves."
Interactive Simulator: Test the Math
Real-World Example: Scenario Breakdown
Examining the real numbers for: Executing an order to purchase 2,000 shares of an equity with an NBBO of $20.00 Bid / $20.01 Ask (1-cent spread)
| Execution Metric | Direct Lit Exchange Midpoint Match User | PFOF Sub-Penny Internalized Client |
|---|---|---|
| Fee / Rate | $0.0035/share DMA rate ($7.00 total) | $0 advertised commission |
| Spread / Buffer | Order routed directly to a lit exchange order book with midpoint routing instructions | Order routed to an off-exchange wholesale market maker under a payment for order flow agreement |
| Execution / Status | Matched against an independent institutional seller at the true $20.005 midpoint ($0.005 price improvement per share) | Wholesaler provided an advertised 'price improvement' of $0.0002 per share, filling the order at $20.0098 |
| Total Cost / Result | Captured genuine midpoint price improvement on a transparent lit exchange | Received token sub-penny price improvement while wholesaler monetized the spread |
How Brokers Weaponize This Term
Review your broker's quarterly SEC Rule 605 execution reports. Look at the 'Price Improvement Percentage'. If your average price improvement on liquid stocks is less than 10% of the prevailing quoted spread, your broker's wholesalers are using sub-penny skims to capture spread revenue.
Broker Evaluation Matrix
Cole Approves
Interactive Brokers: Routes client orders through its SmartRouting engine to actively hunt for true midpoint executions across lit exchanges rather than sub-penny wholesale internalizers.
Read Audit →Cole Flags / Avoids
Zero-Commission PFOF Apps: Routes 100% of retail market orders to wholesale internalizers that use token sub-penny price improvement to monetize customer flow.
View Trap Details →Frequently Asked Questions
What is SEC Rule 612 (The Sub-Penny Rule)?
Rule 612 prohibits market participants from displaying quotes in increments smaller than $0.01 for stocks priced over $1.00, though wholesale internalizers are legally permitted to execute trades in sub-pennies off-exchange.
Why is the SEC proposing changes to sub-penny rules?
Regulators have proposed reducing the minimum tick size to half-pennies ($0.005) or smaller for liquid stocks, allowing public exchanges to compete directly with off-exchange wholesale internalizers.