Reading the Reference Price and Percentage Parameter That Set a Limit State Band

A halt band is an arithmetic output, not a market judgment

When a US stock stops trading because its quote reached a limit, two numbers produced that outcome, and a securities information processor computed both from a published rule: the National Market System Plan to Address Extraordinary Market Volatility, filed under File No. 4-631 and known as limit up-limit down, or LULD. It takes a Reference Price and a Percentage Parameter, multiplies one by the other, and publishes a Lower Price Band and an Upper Price Band. The halt is downstream of those two inputs.

This is a separate mechanism from the market-wide circuit breakers, measured against the S&P 500 Index rather than a single stock. In the Commission's investor education material, a 7 percent decline is a Level 1 event and a 13 percent decline is a Level 2 event, each halting trading market-wide for 15 minutes when triggered before 3:25 p.m., while a 20 percent decline is a Level 3 event that stops trading for the rest of the day. LULD works one symbol at a time.

Where the Reference Price comes from

The plan bases the bands on a Reference Price that equals "the arithmetic mean price of Eligible Reported Transactions for the NMS stock over the immediately preceding five-minute period (except for periods following openings and reopenings, which are addressed below)". Two things there carry weight: the averaging window is five minutes, and the qualifier Eligible Reported Transactions means "transactions that are eligible to update the last sale price of an NMS Stock."

A five-minute mean is a design choice, and the question of what a window actually measures also drives Two Minutes for Crude, Thirty Seconds for the Index. The mean is slower than the last print and faster than a session average, so after a fast decline it sits above the current quote for as long as the older prints stay in the window. That lag is the mechanism, not a defect.

Openings are handled separately. Except when a Regulatory Halt is in effect at the start of Regular Trading Hours, if the primary listing exchange prints an opening price less than five minutes after the start of Regular Trading Hours, that price becomes the day's first Reference Price; if it does not, the plan falls back to the five-minute mean. If no eligible transactions occurred in the window, the previous Reference Price stays in effect.

The percentage sentence that inverts if you cut it at the word or

Appendix A assigns the Percentage Parameter by tier and by where the Reference Price sits. Tier 1 NMS Stocks get 5% above $3.00 and 20% from $0.75 through $3.00. Tier 2 gets 10% above $3.00 and the same 20% from $0.75 through $3.00, and a further clause provides that the parameter for a Tier 2 NMS Stock that is a leveraged ETP is the applicable figure from those clauses "multiplied by the leverage ratio of such product." Below $0.75 both tiers share one sentence, and that sentence is where careless quoting does damage.

The text reads: "The Percentage Parameters for Tier 1 NMS Stocks with a Reference Price less than $0.75 shall be the lesser of (a) $0.15 or (b) 75%." The Tier 2 sentence is identical apart from the tier name. Quote only clause (a) and a $0.10 stock gets a band of plus or minus $0.15, putting the lower band below zero. Quote only clause (b) and a $0.60 stock gets plus or minus $0.45 instead of plus or minus $0.15, a band three times too wide. The word or is the rule, not decoration.

The two clauses cross at one price. Setting 0.75 times the Reference Price equal to $0.15 gives $0.20. At $0.05 the 75 percent term is $0.0375; at $0.10 it is $0.075; at $0.20 it equals the $0.15 cap. Above $0.20 the dollar cap is the smaller term and therefore the operative one, so a $0.40 stock is banded from $0.25 to $0.55 rather than from $0.10 to $0.70.

Concrete Framework for checking a band by hand

Take a Reference Price of $4.00 and work the cases a single symbol can pass through in one session. A Tier 1 NMS Stock at midday carries 5 percent, so the band runs $3.80 to $4.20 and the full width is $0.40. A Tier 2 NMS Stock at midday carries 10 percent, giving $3.60 to $4.40 and a width of $0.80.

Now add the clock. Section V doubles the parameters "Between 9:30 a.m. and 9:45 a.m. ET, and 3:35 p.m. and 4:00 p.m. ET, or in the case of an early scheduled close, during the last 25 minutes of trading before the early scheduled close". The Tier 1 name in the closing period is therefore banded at 10 percent, $3.60 to $4.40, a width of $0.80, which is exactly the midday band of the Tier 2 name. The Tier 2 name in the closing period is banded at 20 percent, $3.20 to $4.80, a width of $1.60. Drawn on a price axis running $3.00 to $5.00, the four bands nest around the same $4.00 center.

A third multiplier appears rarely. If the primary listing exchange cannot reopen a stock after a pause because of a systems or technology problem, "the Processor will calculate and disseminate Price Bands by applying triple the Percentage Parameters set forth in Appendix A for the first 30 seconds such Price Bands are disseminated." The same Tier 2 name would show a 30 percent parameter for half a minute, banded $2.80 to $5.20.

Why the band sits still while the price moves

Bands often fail to update on every print, and Section V says why. The processor runs a Pro-Forma Reference Price continuously, and the plan states that "If a Pro-Forma Reference Price has not moved by 1% or more from the Reference Price currently in effect, no new Price Bands shall be disseminated, and the current Reference Price shall remain the effective Reference Price."

With a Reference Price of $50.00 and a Tier 1 parameter of 5 percent, the band is $47.50 to $52.50, and the pro-forma value must reach $50.50 or $49.50 before anything changes. A move to $50.40 leaves the band untouched. Once the threshold is crossed, at a pro-forma value of $50.60, the new band is $48.07 to $53.13.

A second brake follows: the plan adds "provided, however, that each new Reference Price shall remain in effect for at least 30 seconds." A Reference Price that has just refreshed cannot refresh again for half a minute, however violently the tape moves inside that window. Both provisions explain a band that looks stale while being entirely correct.

Fifteen seconds, five minutes, and the final ten

A Limit State is a quote condition rather than a price condition. It begins when the National Best Offer equals the Lower Price Band without crossing the National Best Bid, or when the National Best Bid equals the Upper Price Band without crossing the National Best Offer. On entry the processor flags the quotation as a Limit State Quotation and stops publishing updated Reference Prices and Price Bands.

The exit condition is a timer with a size test attached. Trading leaves the Limit State if, "within 15 seconds of entering the Limit State, the entire size of all Limit State Quotations are executed or cancelled." Otherwise the primary listing exchange declares a Trading Pause, during which "No trades in an NMS Stock shall occur during a Trading Pause, but all bids and offers may be displayed." That is why a paused symbol still shows a moving quote.

Five minutes after declaring a Trading Pause, absent a Regulatory Halt, the primary listing exchange reopens using its own procedures, and the pause ends when it reports a Reopening Price. The plan adds that "Trading centers may not resume trading in an NMS Stock following a Trading Pause without Price Bands." Late in the day the sequence changes: for a stock paused inside the final ten minutes of Regular Trading Hours, the exchange "shall not reopen trading and shall attempt to execute a closing transaction using its established closing procedures."

What feeds the band is not what the band binds

Here is the asymmetry that catches people. The Reference Price averages only Eligible Reported Transactions, which the plan ties to transactions eligible to update the last sale price. Odd-lot executions are reported and add to consolidated volume without setting the last sale, a distinction traced in An Odd Lot Prints, Adds to Volume, so they generally do not move the average that sets the band.

That does not mean odd lots escape the band. Section VI carves out from the trade-prevention duty "any transaction that both (i) does not update the last sale price (except if solely because the transaction was reported late or because the transaction was an odd-lot sized transaction), and (ii) is excepted or exempt from Rule 611 under Regulation NMS shall be excluded from this limitation." Three constraints sit in that one sentence. The carve-out needs both conditions, joined by and. The parenthetical after except pulls odd lots and late reports back inside the limitation. And single-priced opening, reopening and closing transactions on the primary listing exchange are excluded separately, in their own sentence.

Cut at the word except and you would conclude odd lots are exempt from the bands. Cut at and and you would conclude anything not last-sale eligible is exempt. Both readings are wrong, and both surface only when a fill arrives outside the band you expected.

Tier 1 is two indexes plus a volume-screened ETP list

Tier 1 is not a judgment about quality. Appendix A defines it as the stocks in the S&P 500 Index and the Russell 1000 Index plus the exchange-traded products on Schedule 1, and Schedule 1 is rebuilt on a fiscal half-year cycle from notional consolidated average daily volume, with leveraged products excluded from eligibility. The threshold is stated plainly: "ETPs, including inverse ETPs, that trade over $2,000,000 CADV will be eligible to be included as a Tier 1 NMS Stock."

Everything else falls to Tier 2, with a carve-out inside the definition: "Tier 2 NMS Stocks shall include all NMS Stocks other than those in Tier 1, provided, however, that all rights and warrants are excluded from the Plan." Drop the clause after provided, however and you would expect bands on instruments the plan does not cover.

These boundaries have been contested. In an order dated September 16, 2024, the Commission disapproved the Twenty-Third Amendment, which would have assigned exchange-traded products other than single-stock products to Tier 1 and narrowed the bands on a large population of thinly traded funds. The order found that "the Participants' Analyses do not provide sufficient detail and specificity concerning these securities for the Commission to make an affirmative finding that the Proposed Amendment meets the standard for approval." So an obscure fund can sit in Tier 2 at 10 percent while a large-cap common stock sits in Tier 1 at 5 percent, and that gap reflects a volume test rather than an assessment of the fund.

Where This Doesn't Apply

These bands are calculated and disseminated during Regular Trading Hours. Sessions outside those hours are not covered by the mechanism as approved, which is why the plan participants filed a Twenty-Seventh Amendment, published for notice and comment in the Federal Register on June 4, 2026, proposing separate overnight price band protections with their own reference prices and no automatic trading pauses. Whether that filing or a later one is in effect on a given date has to be checked against the docket for File No. 4-631.

The plan also does not reach instruments outside the definition of NMS Stock, and it expressly excludes rights and warrants. It says nothing about futures, which run on exchange-level daily price limits set in the contract specifications. It is separate again from the market-wide circuit breakers keyed to the S&P 500 Index at the Level 1, Level 2 and Level 3 thresholds described earlier.

None of this is a statement about where a price should go. A band is a boundary on printable and displayable prices, not a forecast, and a stock can sit inside its band all session while moving a great deal, or pause repeatedly while moving very little.

What Would Invalidate This

Every quotation above is taken from the plan text in Exhibit A to Release No. 34-80203, the plan is amended often, and the arithmetic above rests on text the participants can propose to change. An approved amendment to Appendix A altering any tier percentage, the $3.00 or $0.75 breakpoints, or the lesser-of sentence below $0.75 would change every worked example here. An approved amendment to the doubling windows, the 1 percent update threshold, the 30-second minimum, or the 15-second Limit State timer would change the timing discussion. An approved overnight amendment would extend band protection into sessions treated here as uncovered.

One boundary remains untested here: minimum pricing increment changes adopted for quoting can interact with how band values are expressed on a feed, and vendors differ in how they display a band falling between displayable increments. If a feed disagrees with hand arithmetic by a fraction of a cent, check the vendor display first, then the Reference Price the vendor used, then the tier assignment, because a different Reference Price is the more common cause than a different rule.

References: U.S. Securities and Exchange Commission, Plan to Address Extraordinary Market Volatility, Exhibit A to Release No. 34-80203; Federal Register, Order Disapproving the Twenty-Third Amendment to the National Market System Plan To Address Extraordinary Market Volatility; Limit Up-Limit Down Plan, Plan Amendments; U.S. Securities and Exchange Commission, Investor.gov, Stock Market Circuit Breakers.