SPX, SPY, ES — Three Prices for One Index

Scope: a structural explanation of the instruments themselves. It presents no new measurements; the measured claims it leans on are linked to the notes that measured them.

The S&P 500 quotes as three different numbers at once: SPX, the index itself; SPY, the fund that holds its stocks; ES, the futures contract on it. They are not three markets. They are one underlying wearing three wrappers — a measurement, an ownership claim, and an agreement — and each pair is tied together by its own mechanism. The Nasdaq-100 repeats the entire structure as NDX, QQQ and NQ. This note walks the triangle one question at a time.

What exactly is each of the three?#

The distinction that matters most: SPX is a measurement, not a thing. Nobody buys SPX. Everything "priced off SPX" — SPY, ES, SPX options — is an instrument built beside the measurement, held to it by arbitrage.

Why are the three numbers different at the same moment?#

Because each wrapper carries its own adjustment, and none of the adjustments is mispricing.

ES vs SPX differ by the basis — roughly the cost of carrying the position to expiry (financing) minus the dividends a stockholder would collect that a futures holder does not. The gap shrinks as expiry approaches and resets when the front contract rolls. A futures price above or below the cash index is not a forecast; most of it is arithmetic about interest and dividends.

SPY vs SPX differ by scale and by fund mechanics. SPY was designed to trade near one-tenth of the index level, and drifts slightly around that ratio as dividends accrue inside the fund between quarterly distributions and expenses are deducted. Creation and redemption keep the share price pinned to the value of the basket — to its net asset value, not to SPX divided by ten exactly.

QQQ vs NDX works the same way, at a different (and also slowly drifting) fraction of the index level. NQ vs NDX is the same basis logic as ES vs SPX.

The practical rule: the three agree on shape at all times that all three are printing. The levels are separated by known, boring arithmetic — which is exactly why a level read off one chart should not be dropped unconverted onto another.

Which one is "the real price" overnight?#

The futures — by elimination. Overnight, the stocks are closed, so SPX is not being computed; SPY's extended sessions are open but thin. ES trades on. Whatever happens to the world's pricing of US equities between 8 p.m. and 4 a.m. shows up in exactly one liquid place, and by morning the other two wrappers open where the arbitrage says they must. This is the machinery behind the ETF's overnight "gap" — measured directly in the overnight gap is mostly over before you see it: a median 60% of SPY's gap was already in place at 04:01.

It also means "what is the S&P doing right now?" has a different honest answer at 2 a.m. than at 2 p.m. At 2 a.m. the answer is an ES-implied level — the futures price with the basis backed out — because the index itself is asleep.

Why do SPX options and SPY options both exist?#

Because the two wrappers serve different holders, and their options inherit the differences. SPX options are written on the measurement itself: there are no shares to deliver, so they settle in cash, and the contract is sized off the full index level. SPY options are written on the fund: exercise delivers actual shares, and the contract is sized off the ETF's price. One index, two chains — with sizes roughly a factor of ten apart, different settlement styles, and different tax and exercise mechanics that belong to an options guide rather than this note.

What matters here is the structural point: both chains are positioned against the same underlying. Dealers who carry either book hedge in the same places — the futures and the ETF — which stitches the options markets into the same arbitrage web as everything above.

What should a reader take from this?#

• SPX is the reference; SPY and ES are instruments held to it by two different mechanisms — net-asset-value arbitrage and basis arbitrage.
• Same shape always; different levels by construction. Convert before comparing, or compare structure only.
• Overnight there is only one live wrapper. The morning gap in the others is the night's pricing arriving late, not new information at the open.
• NDX / QQQ / NQ is the same triangle with different tickers.

Related reading#

• How ETFs and futures track the same market — the two arbitrage chains in detail.
• When futures actually trade — the contract clocks that make the futures the overnight wrapper.
• Do SPY and QQQ move together? — the co-movement between the two families, measured.