An options spread — buying one strike and selling another — looks like two trades. But your broker sends it as a single combination order priced at one net number: a net debit if you pay to open, a net credit if you collect. Both legs fill together or not at all.

The net price is what you trade

If you buy the 741 call for 1.30 and sell the 743 call for 0.70, you don't manage two prices — you enter one order at a net debit of 0.60. The exchange's combo book matches the whole package. Your limit is on the spread, not on either leg individually.

Why this protects you: leg risk

Imagine filling the two legs separately. You get long the 741 call, and before you can sell the 743, the market jumps — now the short leg is worse, or you're sitting exposed on just one side. That's leg risk, and on a fast 0DTE tape it's a real way to get hurt. Combo pricing eliminates it: you're never half-in a spread you meant to enter whole.

A spread is one instrument, not two trades. The combo fill is why you can't get stranded on a single leg.

The trade-off on 0DTE

Combo orders are safer but can be harder to fill at a good price, because the market maker has to price the whole package — and on thin same-day strikes that package can be wide. That's part of why many scalpers prefer a single option over a spread for fast trades: cleaner, quicker fills. When a spread does make sense, let the combo order do its job and don't try to leg in by hand.