Bid-Ask Spread Compression: How Sub-Pennying Affects Execution

The bid-ask spread is the gap between the highest price a buyer is willing to pay (the bid) and the lowest price a seller is willing to accept (the ask). This spread is the primary measure of market liquidity; a narrow spread indicates a highly liquid market where assets can be bought and sold with minimal friction [5].

As noted by Ryan O’Connell, CFA, execution costs like the spread can silently erode 1-3% of a portfolio’s annual returns, effectively reducing the realized alpha of even the best-constructed strategies.

Sub-pennying refers to the practice where market participants, typically automated wholesalers or high-frequency traders (HFTs), provide “price improvement” by quoting or executing trades in increments smaller than $0.01.

SEC Rule 612 and the “Two-Tier” Market

To understand why this happens, we must look at SEC Rule 612, also known as the Sub-Penny Rule. Established in 2005, this rule prohibits public exchanges (like the NYSE or Nasdaq) from accepting or displaying quotes in increments smaller than a penny for stocks priced over $1.00 [3].

However, a loophole exists: while exchanges cannot quote in sub-pennies, off-exchange venues—such as Internalizers and Dark Pools—can execute in sub-pennies. This creates a two-tier market structure: 1. The Lit Market: Retail and institutional orders sit on public exchanges in $0.01 increments. 2. The Dark Market: Wholesalers can “step ahead” of these public limit orders by offering a fraction of a cent better (e.g., $10.001 instead of $10.01).

While “price improvement” sounds beneficial, the impact of sub-pennying on execution is a double-edged sword.

1. Artificial Spread Compression

Sub-pennying allows wholesalers to provide a slightly better price than the National Best Bid and Offer (NBBO). For a retail trader buying 1,000 shares, a price improvement of $0.001 saves only $1.00. However, this marginal improvement allows the wholesaler to capture the trade flow before it ever reaches the public exchange.

2. Disincentivizing “Lit” Liquidity

When high-frequency players can jump in front of a public limit order by offering a minuscule price improvement, it discourages traders from placing limit orders on public exchanges. If you place a buy order at $50.00 and an HFT steps in at $50.001, your order may never get filled, even if the stock stays at that level. This behavior is a common frustration discussed in trader communities on Reddit, where users often note that their orders are “ghosted” by algorithms jumping the queue.

3. Impact on Volatility and Penny Stocks

In the world of low-priced stocks, the spread is often a much larger percentage of the total share price. According to PennyStocks.com, wider spreads in penny stocks signal lower volume and higher risk, but they also provide more room for sub-penny “scalping” by market makers. This can lead to rapid price fluctuations that catch retail traders off guard.

To maintain your edge, you must adapt your execution strategy to account for these microscopic price games.

• Use Limit Orders Wisely: Market orders are highly susceptible to being filled at the “ask” in a way that favors the wholesaler. Limit orders give you control, but you must be aware that in a sub-penny environment, being “at the bid” does not guarantee a fill [4].

• Monitor the Midpoint: Aim to execute trades at the midpoint of the bid-ask spread. Many modern brokers offer “Midpoint Peg” orders that automatically adjust to stay between the spread, neutralizing some of the disadvantages of sub-penny stepping.

• Psychological Discipline: Execution isn’t just about math; it’s about temperament. As discussed in our guide on Trading Psychology: How Fast and Slow Thinking Affects Trades, rushing to hit the “market buy” button during high volatility often plays right into the hands of HFT algorithms waiting to capture the spread.

• The Spread is a Cost: Every trade begins with a “negative” return equal to the bid-ask spread. Minimizing this is critical for long-term Alpha.

• Sub-Pennying is Selective: Off-exchange wholesalers use sub-penny increments to jump ahead of public limit orders, a practice legal under current SEC loopholes [3].

• Price Improvement vs. Execution Certainty: While sub-pennying offers a slightly better price, it can reduce the likelihood of your limit orders being filled on public exchanges.

• Liquidity Matters: Highly liquid ETFs (like SPY) have spreads as low as 1-2 basis points, making the impact of sub-pennying negligible compared to low-volume penny stocks [1].

Action Plan for Traders

1. Analyze Your Fills: Check your trade confirmations for “price improvement.” If you are consistently getting $0.0001 better than the NBBO but missing large moves, reconsider your order routing.
2. Avoid Peak Volatility: Spreads widen during the market open and close. Trade during the middle of the day when liquidity is most stable to find tighter spreads.
3. Refine Your Strategy: If you are participating in funded accounts or evaluations, execution quality is paramount. See our insights on Prop Firm Evolution to understand how slippage impacts your ability to pass strict trading rules.

Final Thought: Sub-pennying is a symptom of a highly fragmented market. While it offers a veneer of “better prices,” the savvy trader knows that execution quality is measured by more than just a tenth of a cent—it’s measured by the ability to enter and exit positions reliably without being outmaneuvered by the machine.