A spread is two options of the same type with different strikes. That is the entire mechanic. The reason spreads are the default 0DTE structure is risk: max loss is bounded by the spread width minus the credit (or simply the debit paid), and you know the exact dollar figure before you click submit.

This cluster explains the four foundational spreads — bull put credit, bear call credit, bull call debit, bear put debit — and the regime in which each one is appropriate. After the foundations, you can move to iron condors and iron butterflies, which are two-sided spreads.

If you can only execute one structure on 0DTE, make it a defined-risk spread. Naked premium selling on same-day expiration is how accounts disappear.