Iceberg orders, reserve and hidden orders: liquidity you cannot see
Reserve orders show part of their size, non-displayed orders show none, and midpoint orders trade inside the spread. Refills and print-versus-display gaps are clues to hidden size, not proof, and they come before breaks and holds alike.
Start in plain English
An iceberg order shows a small part of its size on the book and keeps the rest hidden, refilling the visible part each time it trades. Exchanges call it a reserve order. Other orders show nothing at all (non-displayed orders), and some rest at the midpoint of the spread. For a small-cap scalper, they answer an uncomfortable question: is there more size at this price than the screen shows?
The core idea: hidden size leaves clues, and the clues are ambiguous. When more shares print at a price than were ever displayed there, something unseen took part. It may be one reserve order, several traders joining in turn, or non-displayed orders. And the same clue appears before the level breaks and before it holds. Spotting an iceberg tells you about the past minute, not about which side runs out first.
"Hidden orders" is the same question asked another way, so this lesson covers both. The How to read Level 2 lesson in US Day Trading Fundamentals covers the displayed book.
Two paths from the same start
All prices, sizes and identifiers in this example are Illustrative. Calloway Fiber and every identifier below are fictional.
The setup at 11:02:30 ET. Calloway set a high of day at $6.10 at 10:51. Since 11:00:00, 24,000 shares have printed at $6.10 while EXCH1 displayed 1,000 shares there each time; the displayed size reappeared 22 times. Another 3,200 shares printed at $6.095, the midpoint of the spread. The trader suspects a seller's reserve order at $6.10 and expects it to run out.
Calloway Fiber (fictional), Level 2 at 11:02:30, ET (Illustrative)
| Side | Price | Size | Source |
|---|---|---|---|
| ask | 6.10 | 1,000 | EXCH1 |
| ask | 6.11 | 800 | ECNB |
| ask | 6.12 | 2,000 | EXCH2 |
| ask | 6.15 | 3,000 | EXCH1 |
| bid | 6.09 | 1,500 | EXCH2 |
| bid | 6.08 | 900 | ECNB |
| bid | 6.07 | 2,500 | EXCH1 |
| bid | 6.05 | 4,000 | MMQD |
Identifiers are fictional. Since 11:00:00, 24,000 shares printed at $6.10 while EXCH1 displayed 1,000 shares there each time; the displayed size reappeared 22 times. 3,200 shares printed at $6.095. High of day $6.10 at 10:51.
The plan, written before 11:02:30. Buy with a marketable limit order at $6.10, filled at $6.10, taking shares from the refilling offer. Stop $6.03. Target $6.24 (2R). Time stop 11:15. Planned risk $0.07 per share. Size: $49 ÷ $0.07 = 700 shares. Costs: $0.005 per share round trip (Illustrative).
Path AThe reserve runs out
The $6.10 offer refills twice more, then does not come back. The $6.11 and $6.12 offers trade within seconds, and price reaches $6.25 at 11:06. The $6.24 limit sell fills.
Path BThe reserve keeps refilling
The offer at $6.10 keeps reappearing for six more minutes; another 66,000 shares print there. Bids at $6.09 and $6.08 thin out as buyers give up. The $6.03 stop triggers and fills at $6.02.
Path CThe offer stays, price waits
The offer refills slowly; neither side presses. Price chops between $6.06 and $6.10. At 11:15 the time stop exits at $6.09.
The review note reads the same for all three: "Bought into a refilling offer at the high; plan followed; 1R planned." Reserve size is hidden by design, so its remaining quantity is the one thing the trade depended on and the one thing nobody could see. The rest of this lesson covers how exchanges handle these orders, what the clues are, and why they mislead.
How exchanges handle hidden size
US exchanges offer several ways to rest size without showing all of it. Names differ by venue; the mechanics are similar (rule text checked 2026-10-05):
| Order type | What shows | Where it is defined |
|---|---|---|
| Reserve (iceberg) | A displayed portion; when it trades, it is replenished from non-displayed size | Nasdaq Rule 4703(h) "Reserve Size"; NYSE Rule 7.31(d)(1) "Reserve Order" |
| Non-displayed limit | Nothing; it rests at its price and trades when a contra order reaches it | Nasdaq Rule 4703(k) (Display attribute switched off); NYSE Rule 7.31(d)(2) |
| Midpoint peg | Nothing; it rests at the midpoint of the best bid and offer | Nasdaq Rule 4703(d); it "may be executed in sub-pennies if necessary to obtain a midpoint price" |
Priority. At the same price, displayed orders trade before non-displayed ones. On Nasdaq, when a reserve order refreshes, the new displayed portion receives a new timestamp, so it goes to the back of the displayed queue at that price; the hidden remainder keeps its place among non-displayed orders. NYSE ranks the displayed portion of a reserve order with displayed orders and the reserve with non-displayed orders.
That has a cost for the hider: hidden size waits behind every displayed order at the same price. A trader using a reserve order to hide trades queue position for discretion.
Why hide at all? A large order shown in full invites other traders to step in front of it or to wait it out. Showing 1,000 shares of 50,000 lets a buyer or seller work the order without advertising its size. The same logic applies to small traders: a 5,000-share bid in a thin small cap can move other traders' behaviour more than its size deserves. Hiding is a trade-off between information leakage and fill speed, not a sign of any particular intent.
The clues, and why they mislead
Clues that hidden size is present at a price:
- Prints exceed display. More shares trade at a price than were displayed there at any moment.
- Same-size refills. The displayed size returns to the same number right after each trade.
- Sub-penny or midpoint prints. Trades at half-cent prices inside a one-cent spread suggest midpoint orders or off-exchange price improvement; see Off-exchange and dark pool prints.
Why each one is weak evidence:
- Several participants at one price produce the same refill pattern. A popular round number or the day's high attracts orders from many traders; each new one appears as "the refill".
- Several venues. Your Level 2 may show some venues and not others. Size that looks hidden may be displayed somewhere your feed does not cover.
- Market makers. A quote refreshed by a market maker looks like a reserve order. Market maker Level 2 covers why.
- Off-exchange trades. Prints reported away from the exchanges can occur at the same price without touching the displayed book at all.
No clue tells you the remaining size, which is what Path A and Path B depended on.
Observe vs. infer
Go back to 11:02:30 in the Calloway example (fictional; all figures Illustrative).
| What the screen shows (observed) | What a trader might infer | Why it is not established |
|---|---|---|
| 24,000 shares printed at $6.10 while 1,000 were displayed | "A big seller has an iceberg at $6.10" | Fits one reserve order, several sellers joining, or a market maker refreshing |
| The $6.10 offer reappeared 22 times at 1,000 shares | "It is about to run out" | Remaining reserve size is not displayed anywhere |
| 3,200 shares printed at $6.095 | "Hidden buyers are at the midpoint" | Midpoint prints fit hidden orders on either side and off-exchange price improvement |
| $6.10 is the day's high | "Breaking it starts a run" | A high is a past price; the breakout is a separate question |
Only the left column was usable at 11:02:30.
Odds box
Slightly positive on purpose, so it reads as neither a warning nor a promise. A few more Path B results, or two cents more slippage on every stop, take it below zero (computed from the assumed split, Illustrative). Your own count is the only one that includes your fills.
What shifts the odds
These are hypotheses to record, not findings:
- Prints-to-display ratio. 24,000 printed against 1,000 displayed vs. 3,000 against 1,000.
- Refill speed. Immediate same-size refills vs. delayed, varying sizes.
- Where the level sits. The day's high, a round number, a premarket level.
- What the other side does. Bids stepping up under the offer vs. bids fading.
- Time of day and spread.
Cost of being wrong
Using the Calloway plan at 700 shares (Illustrative):
| Outcome | Exit | R after costs | Dollars after costs |
|---|---|---|---|
| Path A: target | $6.24 | +1.93R | +$94.50 |
| Path B: stop, 1¢ slippage | $6.02 | −1.21R | −$59.50 |
| Path C: time stop | $6.09 | −0.21R | −$10.50 |
| Path B, bids pulled, 5¢ slippage | $5.98 | −1.79R | −$87.50 |
Being right about the iceberg's existence and wrong about its size costs the same as being wrong about both.
How many trades before this means anything
With 20 entries into refilling offers and 7 reaching the target (35%), the 95% interval runs from about 18% to 57% (Wilson, computed on hypothetical counts). Entries at the same level on the same morning share one hidden order, so they count for less than they appear to. See How many trades before your numbers mean anything.
What this lesson does not claim
- It does not claim refills reveal who is trading or how much is left.
- It does not claim breaking a refilling offer leads anywhere in particular. The odds box is Illustrative.
- Rule descriptions summarize exchange rule text checked on 2026-10-05.
- Calloway Fiber, its identifiers and figures are fictional and Illustrative.
Go deeper
- FennEdge first. FennEdge does not offer Level 2 depth as of 2026-10-05. The Scanner (paid: Plus, Pro, Max) covers rapid rises and drops, regular-session highs and Nasdaq-listed LULD halts.
- Your own hidden orders. A non-displayed order waits behind displayed orders at its price, so it fills later or not at all. Order routing covers queue position.
- Retail price improvement. Some exchange programs let liquidity providers offer sub-penny improvement to retail orders; see Tick size.
- Related: Absorption, the same clue on the bid side; Spoofing, the opposite case of displayed size that disappears.
Exercises
Six exercises follow this lesson: three concept checks, two replays of a static Level 2 snapshot (classify observation vs. inference; then pick every outcome consistent with the book at 11:02:30 before Path B is revealed), and one scenario on reading a refilling offer. Exercises, answers and progress are available after signing in.
Primary sources
- Nasdaq. Equity 4 rules: Rule 4703 (Order Attributes: Reserve Size, Display, Pegging) and Rule 4757 (Book Processing).
- NYSE. Rule 7.31, Orders and Modifiers.
- NYSE Arca Rule 7.31-E: SEC Release No. 34-98899, Federal Register, November 15, 2023.
Sources checked 2026-10-05. Calloway Fiber and all quotes, identifiers, prints, outcome splits and costs in this lesson are fictional and Illustrative. This lesson is education, not a recommendation to buy or sell any security. Past patterns do not indicate future results.