Box spread

A box spread combines a long call spread and a long put spread on the same two strikes and expiry. Its payoff at expiry equals the distance between the strikes, whatever the underlying does. It carries no market exposure, which turns it into a lending or borrowing instrument.

How it works

With strikes K1 below K2, the long box is: buy the K1 call, sell the K2 call, buy the K2 put, sell the K1 put. At expiry it pays K2 − K1. Its fair price today is that amount discounted at the market rate to expiry. Buying the box below that amount means lending at the implied rate, and selling it means borrowing. Because the counterparty is the clearing house, the implied rate tends to sit close to secured funding levels. The trade only works cleanly on European-style, cash-settled options such as SPX, where no leg can be assigned early.

Example

Hypothetical: on index options with a $100 multiplier and one year to expiry, a box on the 4,000 and 5,000 strikes pays 1,000 points, or $100,000. If it trades at 960 points, or $96,000, the implied one-year rate is 1,000 / 960 − 1, about 4.17%. The buyer lends $96,000 and receives $100,000 at expiry.

On the desk

Traders compare the box rate with other funding sources across maturities. On American-style options, early assignment of a short leg breaks the structure, which is why the trade belongs on European-style contracts.