Order types

Iceberg, hidden, pegged, conditional, IOC/FOK/post-only. Each order type exists for a reason, and HFTs both use them and detect them in others’ flow.

See it move

Why are there so many order types?

Because a plain limit order leaks three things you might not want to give away: your size, your price intention, and your urgency. Advanced order types let you manage each: conceal size (iceberg or hidden), automate your price (pegged), pre-arm a trigger (stop or conditional), or control immediacy and fees (IOC, FOK, post-only). Each trades one cost for another, and each leaves a different footprint on the book.

The intuition is that posting a big visible limit order is like showing your whole hand. The book is read by everyone: a large resting bid announces "there is a buyer of size here", inviting others to trade ahead of you (quote-matching) or to fade you. Order types are the toolkit for revealing less, or reacting faster, at a price. They act on three axes: visibility (iceberg, hidden), price-tracking (pegged), and time or condition (stop, IOC, FOK, post-only, GTC or day). A single order can combine flags (a hidden, post-only, day order is perfectly ordinary) so the catalogue below is a set of dials, not a menu of mutually exclusive choices. The table summarises the catalogue; the sections that follow work through each in turn. Use the order-type playground above to place each type and watch what the book reveals.

Iceberg and hidden orders: concealing size

An iceberg (reserve) order displays only a small peak of its true size; when the peak fills, the venue automatically replenishes it from the hidden reserve until the full quantity is worked. A hidden order shows nothing at all on the feed but still rests and trades. Both let large orders participate without advertising size, at the cost of worse queue priority for the concealed portion.

With an iceberg / reserve order you post, say, 10,000 lots with a 500-lot display peak. The book shows 500; when it fills, another 500 surfaces, and so on until the reserve is exhausted. The historical tell is a level that keeps refilling after repeated fills (a signature that tape-readers and pingers learned to spot) which is why venues now often randomise the replenishment size and timing to defeat exactly that detection. A hidden order is fully invisible to the public feed but still matchable; the trade-off is that hidden orders typically yield priority to displayed orders at the same price (you fill only after the visible size is exhausted) and may forfeit the maker rebate, because the venue prices the concealment. On many venues, non-displayed liquidity sits inside the spread, for example as midpoint pegs in dark pools. The HFT angle cuts both ways: liquidity providers use these to work size without leakage, while predators try to detect the hidden quantity by sending small pings (tiny IOCs) to see what trades against nothing visible, the recognition-only subject of pinging & sniffing. Treat detection as defence, never as an operational how-to.

Pegged orders: tracking the price automatically

A pegged order automatically re-prices to track a reference. A primary peg sits at (or one tick inside) the same-side best quote; a midpoint peg tracks the mid; a market peg tracks the opposite-side touch. The venue moves it for you as the market moves, so you hold a relative price without resubmitting, useful for passive participation without constant cancel and replace.

A primary peg tracks your own side's best quote (a buy pegs to the best bid) so you are always at the touch as it moves, often with an offset such as one tick more passive. A midpoint peg rests at the midpoint of the NBBO: a non-displayed "split the spread" order, common in dark and midpoint venues. You give up nothing of the spread to the other side and capture half of it, with a fill happening only when someone else also accepts the mid. A market peg tracks the opposite side, more aggressive, used to stay marketable. HFTs care about pegs because they offload the cancel-and-replace work of staying at a relative price onto the venue, cutting message traffic and sometimes beating self-management given latency, and because midpoint pegs are a primary way to source or provide hidden liquidity at the mid. The catch is that a peg's behaviour is fully specified by the venue, so it is predictable, and predictable orders can be modelled and anticipated by others.

Conditional and time-in-force: stop, IOC, FOK, post-only

Conditional orders activate on a trigger: a stop becomes a market order once a stop price trades; a stop-limit becomes a limit order. Time-in-force and execution flags govern lifespan and maker/taker status: IOC fills what it can now and cancels the rest; FOK fills completely now or not at all; post-only rejects or re-prices rather than crossing, guaranteeing maker status.

A stop / stop-limit is not in the book until its trigger price trades; it then releases as a market (stop) or limit (stop-limit) order, used for risk exits and breakout entries. The honest caveat: a stop becomes a market order, so in a fast move it can fill far through the stop price (slippage) while a stop-limit caps the price but risks no fill at all. Clusters of stops are a known liquidity feature, and where they are deliberately triggered the behaviour edges into momentum ignition (recognition only). IOC (immediate-or-cancel) takes whatever is available now at your limit and cancels the remainder: the workhorse of liquidity-taking and of pinging, since a one-lot IOC probes for hidden size and leaves no resting footprint. FOK (fill-or-kill) is all-or-nothing, immediately, used when a partial fill is useless, for example one leg of an arbitrage that only works in full size. Post-only ensures the order never crosses the spread, so it always rests as a maker, earning the rebate and never paying the taker fee; if it would be marketable on entry, the venue rejects or re-prices it. It is essential to a maker-only strategy (maker-taker) and ubiquitous on crypto venues. And the plain lifespan flags, GTC, day, GTD (good-til-cancelled, day, good-til-date), are the boring but necessary defaults.

Worked example

Take a synthetic book, as of 2026; reproduce it in the playground above. The best ask is 50.01 with 200 lots displayed; a hidden 1,000 lots also rests at 50.01; and the mid is 50.005. Everything here is synthetic, and the point of the example is that the public book is not the true book.

An iceberg buy of 5,000 with a 500-lot peak at 50.00 shows 500 on the feed; as sellers hit it, it refills to 500 nine more times until the 5,000 is worked. A tape-reader sees the level "never run out" (the iceberg tell) unless the venue randomises the peaks (say 380, 540, 470, …), which defeats the simple detector. A midpoint-peg buy rests hidden at 50.005, invisible on the lit feed, and fills only when a sell also accepts the mid, capturing half the spread versus crossing to 50.01. A post-only buy at 50.01 would be marketable, since it crosses the 50.01 ask, so the venue rejects or re-prices it to 50.00, guaranteeing you stay a maker rather than accidentally paying the taker fee.

That two-ping sequence is the detection logic pinging & sniffing describes: recognition only, and note that venues randomise and surveil for exactly this. Finally, an FOK buy of 1,000 at 50.01 meets only 200 displayed but 1,000 hidden also resting at 50.01, so whether it fills depends entirely on whether the engine matches the hidden size: the cleanest illustration that the public book is not the true book. The numbers here are synthetic and rounded; exact order-type semantics (the priority of hidden versus displayed, peg offsets, post-only re-pricing, iceberg randomisation) are venue-specific and must be verified against the venue's order-type spec and dated.