A block trade is a single large options order, typically negotiated away from the public order book and printed to the tape afterwards. Where a sweep signals urgency, a block signals size.
Why blocks trade off-exchange
Suppose you want 5,000 contracts. Posting that on the public book announces your intention to everyone. The market moves away before you fill, and you end up paying materially worse than the price you saw.
Instead the order is negotiated privately — through a broker, an upstairs desk, or a dark venue — with a counterparty willing to take the whole size at one price. Once agreed, the trade prints publicly. You see it after the fact.
That is the trade-off: the block gets done at a single clean price, and the market learns about it only once it cannot do anything about it.
Blocks vs sweeps
| Sweep | Block | |
|---|---|---|
| Routing | Split across many exchanges | Single negotiated trade |
| Signals | Urgency | Size |
| Pricing | Takes whatever is there, crosses the spread | One agreed price, often at the mid |
| Timing | Immediate, aggressive | Arranged, deliberate |
A sweep says I need this now. A block says I need a lot of this. They are different messages, and a trader who reads every large print as the same thing is losing information.
Reading direction is harder
Blocks are frequently negotiated at or near the midpoint, which strips out the bid/ask inference that makes sweeps readable. A block printing at the mid tells you size and nothing about aggression.
Worse, blocks are disproportionately likely to be institutional structure rather than a directional view — a hedge against an equity position, a collar, one leg of a spread, or a roll. Large call block does not mean bullish. It may well mean someone just hedged a short.
Dark pool and ISO prints
Some feeds tag off-exchange and intermarket sweep order prints separately. These are worth isolating: they represent flow that never appeared on the live exchange tape, so anyone watching only the visible book missed it entirely.
Using it
Blocks are best read as evidence that a serious participant took a position — which is genuinely useful — while remaining agnostic about what the position is for. Combine them with dealer positioning to ask the more useful question: does this size stack on top of existing gamma at these strikes, or does it fade it? The answer tells you more than the print alone ever will.