Open SPX in the Terminal and the number in the quote bar is not the cash index you see on the news. Scroll to an expiration six months out and the strike where the call and the put are worth the same sits tens of points above it. Neither is a glitch. Both come from the way an index option chain is built, and once you see it you will read every index chain differently.
Stocks have a last trade; the index does not
For a stock, the underlying is a real thing with a real last trade, so every expiration on the chain refers to the same price. A cash index like SPX is a calculation, not a security. Nothing trades at that level, so the data feed does what the last two lessons taught you: it backs the underlying out of the options themselves using parity, strike plus call minus put, once per expiration. What you get is not spot. It is the forward for that expiration, and it is different for every expiration on the chain.
Why the forward climbs with time
Carry. Interest on the index level minus the dividends the index pays. With rates at 4% and a dividend yield of 1.3%, the carry is about 2.7% a year. On an index at 6,000 that is 162 points a year, about 81 points over six months and roughly 13 points a month. So the one-week expiration reads within a point or two of spot, the three-month expiration reads about 40 points above it, and the six-month reads 80 or so above. The quote bar shows the nearest expiration's level, which is why it is close to the cash index but not equal to it.
The $100 call minus the $100 put on an index at $100, at a 4% rate and a 1.3% dividend yield: the forward's lead over spot, discounted to today. About $1.33 at six months and $2.63 at a year. Set the dividend equal to the rate and the line goes flat at zero.
Scale the chart up to an index at 6,000 and the $1.33 at six months becomes 80 points, the gap you see on the chain. There is a practical reason the chain uses this number. Desks that hedge index options do not buy 500 stocks; most hedge with index futures, and a future's price is a forward. For an index book the forward is not a technicality, it is the price of the hedge.
Reading an index chain
- The quote-bar level belongs to the nearest expiration. Treat it as spot.
- Each expiration has its own at-the-money strike, and the further out you go the higher it sits. Find it by looking for the strike where the call mid and the put mid are closest to equal.
- The 0.50 delta call sits near the forward, not near spot. A call struck at spot six months out is slightly in the money and will show a delta above 0.50.
- Expected-move arithmetic from implied volatility should be centered on the forward for that expiration.
- SPX options are European and settle in cash to the index level, so there is no early exercise and parity holds tightly across the whole chain.
Delta of a call struck at today's level, index at 15% volatility. With a year left it is near 0.59, not 0.50, because the strike sits below that expiry's forward. Drag the dividend up to the rate and most of the lift disappears; the small rest comes from volatility itself.
That is the trap in reading a long-dated index chain from spot. The strike at today's level is not at the money for that expiration, and every measure built on it, delta, the expected move, the skew, is shifted by the carry.
SPY is different
SPY is an ETF that trades all day, so its chain references its last trade for every expiration, like any stock. The forward still exists, but it lives inside the option prices rather than in the underlying price. That is why SPY chains look ordinary and SPX chains look shifted, even though they describe the same market.
The maths: one forward per expiration
Each expiration T has its own forward, F(T) = S × e^((r - q) × T). With the index at 6,000, r = 4% and q = 1.3%, the carry rate is 2.7% a year. Three months (T = 0.25) gives 6,000 × e^0.00675 = 6,040.6. Six months (T = 0.5) gives 6,000 × e^0.0135 = 6,081.6. Those are the 40 and 80 points in the text.
The chain works it backwards. From parity, F = K + (C - P) × e^(rT). Say on the six-month chain the 6,080 call mid is $1.60 above the 6,080 put mid. Then F = 6,080 + 1.60 × e^0.02 = 6,080 + 1.63 = 6,081.63. The quick version, strike plus call minus put, gives 6,081.60; the discount factor adds three cents. The data feed runs this for every expiration, which is why an index chain carries a different underlying level for each expiry.
Index dividends are not paid evenly through the year; many companies pay in the same few weeks each quarter. So the real forward curve rises in small, uneven steps rather than as the smooth line in the chart.
Your turn
Open SPX. Read the quote-bar level, then open an expiration about six months out and back out the forward from the strike nearest the money. Notice how far above the quote bar it sits.