A pegged order has its price set automatically by reference to a specified market benchmark, such as a best bid, best ask or their midpoint. Its update policy lets a trading strategy follow the market without manually replacing a fixed price at every quote change. That convenience does not guarantee execution, preserve queue priority or prevent the strategy from following an increasingly unattractive market.
The SEC staff’s Rule 605 FAQ, question 14 describes the benchmark-based mechanism and distinguishes an order that can keep repricing from an immediate-or-cancel instruction. The latter either executes immediately or cancels its unfilled quantity; there is no continuing order to follow later quotes.
Specify what the order follows
“Pegged” names a mechanism, not a complete execution instruction. For the terminology used in Nasdaq Rule 4703(d), the broad reference choices are:
| Peg reference | Buy order follows | Sell order follows |
|---|---|---|
| Primary | Same-side bid | Same-side ask |
| Market | Opposite-side ask | Opposite-side bid |
| Midpoint | Midpoint of bid and ask | Midpoint of bid and ask |
These labels require the venue’s precise reference definition. A local quote and the US national best bid and offer need not coincide. An allowed offset can move an order away from its reference, while a price limit constrains what the trader authorises. Display, routing and lifetime are further instructions.
For a midpoint reference with best bid and best ask , the raw reference is
This midprice is a quote calculation. It is not evidence that another participant will trade there.
A cap can make the midpoint unavailable to the order
Consider an original synthetic midpoint-only buy order with zero offset and a limit price of USD 101.50 per share. Its explicitly stipulated policy is:
- Use the current midpoint only when both reference quotes exist, the bid is strictly below the ask, and the midpoint is at or below the cap.
- Otherwise park the order: it has no executable price. Recheck it at a later valid quote update. Parking here retains the instruction rather than cancelling it.
The toy policy permits equality at the cap. It has no discretion to buy above it and does not replace an impermissible midpoint with the cap.
| Quote state | Reference bid / ask | Raw midpoint | State under this toy policy |
|---|---|---|---|
| Initial | 100 / 102 | 101 | Eligible at 101 |
| Later | 101 / 103 | 102 | Parked: 102 exceeds 101.50 |
All prices are USD per share. The first midpoint is 0.50 below the cap; the second is 0.50 above it. The reference has moved, while the trader’s maximum authorised purchase price has stayed fixed. The later order is not eligible at 102 and is not automatically repriced to 101.50.
Original synthetic quote-state example. Filled and hollow circles distinguish an eligible midpoint from an ineligible raw midpoint; the dashed line is the fixed buy cap. No execution is stipulated.
The example supplies quotes, not a sequence of matching orders, so it establishes no executions. Even the initially eligible price needs compatible selling interest and the applicable allocation conditions before a fill can occur. Repricing, eligibility and execution are separate events.
Carry the venue policy into the model
The parking rule above is a teaching choice. It is not Nasdaq’s universal treatment of a midpoint beyond a limit. Rule 4703(d) states that, at initial entry, a midpoint buy whose limit is below the midpoint is ranked at its limit. It also distinguishes managed and fixed midpoint instructions; specified repricing can receive a new timestamp. Those differences matter to a simulation’s state and priority assumptions. Check the exact venue, protocol and order attributes before translating a peg into code.
A moving peg can reduce the distance between an order and its reference, but it does not establish the reference as fair value. A buy order can follow a rising market after its signal has weakened; a passive fill can still suffer adverse selection. Evaluate the instruction alongside its limit, expiry and cancellation policy.
For the underlying distinction between price protection, completion and confirmed order state, read Market, Limit and Conditional Orders. The public page is a preview; the full lesson requires membership. It develops the order lifecycle rather than a particular venue’s peg implementation.
Sources
- SEC staff, Frequently Asked Questions: Rule 605 of Regulation NMS, question 14, 1 April 2026: benchmark-based pricing and the effect of immediate-or-cancel handling. This is staff guidance about execution-quality reporting.
- Nasdaq Equity 4, Rule 4703(d), Pegging: reference choices, limits and venue-specific update handling, checked 27 October 2026.
- Rishi K. Narang, Inside the Black Box: A Simple Guide to Quantitative and High-Frequency Trading, 2nd edition, Wiley, 2013, Chapter 7, “Aggressive versus Passive”, pp. 121–124, and “Other Order Types”, pp. 124–125: execution uncertainty, priority and order modifiers. The numerical example and parking policy here are original.