Effective Tick Size (Sub-Penny Clustering)
The Formal Definition
The phenomenon where transaction executions cluster at specific sub-penny fractional price increments (e.g., $0.0001 or $0.0050) on off-exchange internalizer ledgers, revealing that institutional wholesalers capture spreads in increments that lit public exchanges are legally prohibited from displaying.
Sub-Penny Cluster Differential = Internalizer Execution Fill ($XX.XX01) - Displayed Lit NBBO Limit Increment ($XX.XX00)
Cole Barrett's Reality Check
The Unvarnished Bottom Line"On a public exchange, a stock trades in whole pennies. But look at the trade tape from a wholesale internalizer, and you will see trades clearing at $50.0023 and $50.0078. That sub-penny clustering proves the game is rigged. Wholesalers trade in fractional cents to step in front of public limit orders by 1/100th of a penny, capturing the trade while public lit exchanges are forced to sit on the sidelines."
Interactive Simulator: Test the Math
Real-World Example: Scenario Breakdown
Examining the real numbers for: Retail market order executing 5,000 shares inside a $30.00 Bid / $30.01 Ask quoted market
| Execution Metric | Lit Exchange Limit Bidder (Penny Bound) | Wholesale Market Maker (Sub-Penny Cluster Trader) |
|---|---|---|
| Fee / Rate | $1.00 fee | $0.00 internal |
| Spread / Buffer | Posted displayed limit bid at $30.00 (Restricted from quoting $30.0001 by Rule 612) | Bought incoming retail sell order off-exchange at $30.0001 |
| Execution / Status | Wholesaler stepped ahead off-exchange; lit order sat unfilled | Stepped ahead of public lit book by a fraction of a cent |
| Total Cost / Result | Denied execution due to statutory tick rules | Monetized sub-penny regulatory arbitrage |
How Brokers Weaponize This Term
Wholesale internalizers lobby aggressively against SEC proposals that would allow lit exchanges to quote in half-penny increments, preserving their exclusive off-exchange sub-penny spread capture monopoly.
Broker Evaluation Matrix
Cole Approves
Interactive Brokers: SmartRouting engine routes orders to lit exchanges and midpoint crossing facilities to capture genuine sub-penny price improvement for customers.
Read Audit →Cole Flags / Avoids
Zero-Fee PFOF Portals: Routes 100% of non-directed retail equity orders to wholesale internalizers that monetize tick-size regulatory arbitrage.
View Trap Details →Frequently Asked Questions
What rule governs sub-penny quoting in the United States?
SEC Rule 612 of Regulation NMS prohibits lit exchanges and market participants from displaying, ranking, or accepting orders in increments smaller than $0.01 for stocks priced at $1.00 or higher.
Why are off-exchange internalizers allowed to execute in sub-pennies?
Rule 612 prohibits *displaying* or *ranking* quotes in sub-pennies, but wholesalers exploit an exception that permits *executing* trades in sub-pennies if it provides price improvement over the displayed lit quote.