Charm exposure — CHEX — measures the hedging that the passage of time forces on market makers. Not a price move. Not a volatility move. Just the clock running down.
An option's delta is a statement about probability: how likely this contract is to matter by expiry. And that probability changes as time runs out, even in a market that does not move an inch.
An out-of-the-money option that has not moved closer to the strike gets steadily less likely to finish in the money. Its delta drifts toward zero. An in-the-money option gets steadily more certain, and its delta drifts toward 1.
Charm is the Greek that measures that drift — how much delta changes per unit of time. It is also called delta decay. Charm exposure (CHEX) is the sum of it across the dealer book: how much re-hedging the clock alone will force into the market.
Not to be confused with theta. Theta is the decay of an option's price. Charm is the decay of its delta. Theta tells you what the contract is worth; charm tells you what the dealer holding it has to do about their hedge. Only one of the two produces orders in the underlying — and it is not theta.
Take the standing position again. Dealers are short puts from investors buying protection, and hedged short the underlying against them.
Now let a quiet week pass with those puts still out of the money. Their deltas decay toward zero. The dealer's short hedge is now too large for a position that is quietly evaporating. To stay neutral, they buy the underlying back — a little at a time, all week.
That is the charm bid: the slow, unglamorous drift higher that so often shows up in a quiet market with no catalyst, and the reason expiry week has a character of its own.
The critical thing about charm is that it accelerates. Delta decay is gentle when there is a month left and violent when there are hours left, because the same uncertainty has to be resolved in a fraction of the time.
This is why Friday afternoons and monthly OPEX have a distinct feel, and why the last two hours of an expiry session can carry flow that has nothing to do with anyone's opinion about the market.
It is also why 0DTE is the most charm-intensive product that exists. A same-day option must resolve its entire delta — all the way to zero, or all the way to one — inside a single session. The re-hedging that a monthly option spreads across weeks gets compressed into a few hours.
Four times a year — the third Friday of March, June, September and December — stock index futures, stock index options and single-stock options all come off the board on the same session. The financial press calls it triple witching (you will also see it called quadruple witching).
Nothing special happens to the Greek on those days. What happens is that they carry the most open interest of the year, and charm flow scales with open interest. More delta sitting on the dealer book to decay means more delta to unwind. It is the same mechanism as any Friday, run at the largest size the calendar offers.
Triple witching has a reputation for chaos it does not really earn. The volume is enormous. The volatility, on average, is not — sell-side studies have repeatedly found intraday ranges on triple-witching sessions running at or below an ordinary monthly expiry.
That is not a paradox once you know the positioning. Enormous open interest concentrated at strikes is precisely the condition that leaves dealers long gamma — and long-gamma hedging pins price rather than propelling it. Big flow, small range. The volume is real; the fireworks are folklore.
And not all of that flow is charm. Index products settle against Friday's opening prints; quarterly index rebalances execute in the closing auction; and a large share of the volume is nothing more exotic than rolling a position into the next contract. Charm is one strand of it — the strand that runs into the close, because SPY, QQQ and single-name options are p.m.-settled and trade right up to the bell. That is the strand CHEX measures.
Gamma is what the market does when price moves. Vanna is what it does when volatility moves. Charm is what it does when nothing moves — because time passing is, by itself, enough to force dealers to trade.
Dark Horse computes the day's net charm exposure and plots it alongside the gamma, delta and vanna panels — so the flow that builds into the close is visible while it is building, not afterward. Charm exposure is included on the Complete tier, on any optionable US ticker.
One honest note, which we would rather state than bury: higher-order exposures like charm are model-derived estimates. They depend on a pricing model and its assumptions in a way that first-order gamma does not. That is true of every provider who publishes them. Read CHEX as positioning context, not as a precise number.
Not investment advice. For educational purposes only. Market maker positioning levels, not trade signals.