A diagonal spread combines two options of the same type but with both different strikes and different expirations. It's a hybrid: like a vertical spread it has different strikes (a directional lean), and like a calendar spread it has different expirations (a time-decay component). Typically you buy a longer-dated option and sell a shorter-dated one at a different strike.
How it works
You own the longer-dated option (your core position) and repeatedly sell shorter-dated options against it at a strike offset in your favored direction. The short option decays faster (harvesting theta), while the long option gives you directional exposure and staying power. It's a way to express a directional view while getting paid for time decay along the way.
Why it's flexible
The diagonal's appeal is flexibility: you can lean bullish or bearish (via the strikes), collect income (via the short leg's decay), and roll the short option repeatedly against a longer-dated anchor — a bit like a covered call but using a long option instead of stock ("poor man's covered call"). The trade-offs are complexity and sensitivity to both direction and volatility.
A diagonal is a directional bet that pays you rent while you wait — at the cost of real complexity.
The honest view
Diagonals are among the more advanced structures — you're managing strikes, two expirations, theta, and vega at once, with a profit profile that shifts as the near leg expires and gets rolled. Powerful for a patient, directional-with-income thesis; easy to mismanage if you don't respect the moving parts. Understand each leg's role before combining them.