Box Spread

A box spread is an options position built from four separate option contracts that, together, lock in a fixed payoff at expiration no matter where the stock price ends up. Because the payoff is fixed and known in advance, the position behaves less like a directional bet and more like a synthetic loan: you either pay money up front and collect a larger fixed amount later, or collect money up front and owe a larger fixed amount later.

It is built by combining two vertical spreads. A vertical spread is just a pair of options of the same type (both calls, or both puts) at two different strike prices but the same expiration. One vertical spread is made with calls (buy a call at one strike, sell a call at a higher strike) and the other is made with puts (sell a put at the lower strike, buy a put at the higher strike). Put together, the ups and downs of the stock price cancel out, leaving a payoff at expiration equal to the difference between the two strike prices, always.

Because the outcome is fixed, the only real variable is the price you pay or receive to enter the box today versus the guaranteed amount you'll get back at expiration. That difference is effectively an implied interest rate. Traders sometimes use box spreads to borrow or lend money synthetically through the options market, arbitraging any gap between that implied rate and prevailing interest rates elsewhere.

The nuance that trips people up is that "riskless" refers to price risk, not to all risk. You still face counterparty and execution risk, the cost of commissions and the bid-ask spread on four separate legs (which can quietly erode the theoretical edge), the possibility of early exercise on the short American-style options before expiration, and the capital tied up until settlement. It is also a strategy that requires enough margin and buying power approval from a broker, since some legs involve selling options.