The diagonal spread

Sell a near-dated option and buy a longer-dated one at a different strike.
A calendar and a vertical in the same position — which is why it is harder to reason about than either.

What it is

Two legs, two strikes, two expiries.
You sell the near-dated option, usually out of the money.
You buy the longer-dated one, usually closer to the money or in it.
Change one date and it is a vertical. Change one strike and it is a calendar. It is both.

What you are actually agreeing to

You are paying for a long option and renting part of it out.
You want the stock to drift towards the strike you sold, but not through it, before the near expiry.
Afterwards you still hold the long leg, and you can sell against it again.
The position’s value at the near expiry depends on volatility, exactly as a calendar’s does.

The trade, written out

Everything above is the idea. This is the position — the lines you would actually send to a broker.

Order ticket

Leg	Action	Qty	Expiry	DTE	Strike	Delta	Price
1	Sell to open	-1	16 Oct 26	45	105 Call	-0.32 (32Δ)	$1.77
2	Buy to open	+1	14 Jan 27	135	95 Call	+0.70 (70Δ)	$9.61
Net	Debit	1 position	16 Oct 26	45	—	+0.37	$7.83

BUY +1 DIAGONAL XYZ 16 OCT 26/14 JAN 27 95/105 CALL @ 7.83 DEBIT

Reading it back

Two different strikes AND two different dates. That is what makes it diagonal.
The near leg is the income.
The far leg is the position, and the larger part of the ticket.
Read the DTE column and the strike column together — either one alone describes a different structure.

The argument

Bull

makes the claim

I get the direction I want, cheaper than owning the stock, and somebody else pays part of the cost every month. When the near option expires I sell another one. It is a covered call where the cover costs a fraction of the shares.

Bear

doubts it

You have two moving parts and you have to be right about the interaction of both. If the stock runs past your short strike quickly you cannot deliver — you have a long option, not shares — and the position you are left holding is not the one you thought you owned.

Ferret

settles it

The measurable version of that disagreement is what happens on a fast move up, how often it happens, and what it costs when it does. Nobody publishes that. It needs per-leg volatility to test properly, which is the same limit the calendar has.

The shape of it

The diagonal spread on XYZ at $100 · 25% volatility · 4% rate · 45 days. Priced from the model, not written by hand.

	At expiry
Debit paid	$783
Maximum profit	$437
Maximum loss	$-783
Breakeven	$99.28

A rising, tilted hill. It leans in the direction of the long leg.
The best outcome is the stock near the short strike at the near expiry.
The shape past the short strike depends on the long leg’s value, which is a model price rather than intrinsic value.
The maximum loss is roughly the debit, but only if the long leg behaves as the model says it will.

Where you actually enter

The chart above is the shape at expiry. It puts the bend right next to the money, which makes the trade look like it is already on the edge. It is not — here is the day you open it.

The same structure set up at 30Δ · 45 days · credit $749. The shaded band is the room between the stock and the strike you sold.

The same price, two different days

	On day one	At expiry
Stock at $106 (the strike you sold)	$195	$476
Stock unchanged at $100	$0	$21

You enter below the short strike, with the position leaning your way.
The day-one line is much flatter than the near-expiry shape.
The interesting part is what you own on the day the near leg dies, and that is not visible on an expiry chart at all.
Which is why a diagonal is best judged as a sequence, not as a single trade.

Different ways to set it up

Three dials, not two: where the short strike sits, how far apart the strikes are, and how far apart the dates are. The panels move the near tenor and hold the rest.

Same delta, four tenors

Days	Short strike	Away from spot	Credit	Max loss	Risk : reward
7	102	-2.0%	$1,000	$-1,000	—
21	104	-4.0%	$861	$-861	—
45	106	-6.0%	$749	$-749	—
90	109	-9.0%	$634	$-634	—

A short near leg means frequent income and frequent decisions.
A longer near leg means fewer of both.
The long leg carries the position across all of them, which is what separates a diagonal from a series of verticals.

Same tenor, three deltas

Delta	Short strike	Away from spot	Credit	Max loss	Risk : reward
40Δ	103	-3.0%	$848	$-848	—
25Δ	107	-7.0%	$714	$-714	—
15Δ	110	-10.0%	$606	$-606	—

A closer short strike collects more and caps you sooner.
A further one collects less and leaves more upside.
It is the same decision as a covered call, made against a long option instead of against shares.

A diagonal is a covered call whose cover is an option. Everything odd about it follows from that one substitution.

What the Greeks are doing

Four questions about the same position, and a fifth that barely applies here.
Every structure answers them differently, which is why this site never writes a general page about the Greeks.

Days left	Delta	Gamma	Theta / day	Vega	Rho
45	+37.1	-1.79	$+1.21	$+8.67	$+18.38
21	+47.4	-2.52	$+1.73	$+11.88	$+17.35
7	+62.2	-1.84	$+0.99	$+15.57	$+16.36

The position at entry — stock $100 · 25% volatility. Per contract, from the model.

Delta — which way you need the stock to go

Positive on a call diagonal — you want it up, gently.
The short leg holds the delta down, and it does so more as the stock rises.

Theta — what time does to you

Positive, from the near leg outpacing the far one.
It is the same engine as a calendar, tilted.

Vega — what a change in fear does to you

Net long volatility, because the far leg has more vega.
So a volatility spike helps — the opposite of every credit structure here.

Gamma — how fast your delta turns against you

Negative near the short strike, and softened by the long leg.
The long leg is a genuine cap on this, unlike on a naked short.

Rho — what a change in interest rates does to you

Bigger than usual, because one leg is months further out.
Still one row.

Theta and gamma are one thing

This is the part that decides whether the strategy works, and it is almost never put plainly.

You cannot be paid theta without being short gamma. They are not two features of the trade. They are the rent and the risk on one lease.

The same position, three points in its life

Nothing about the position changes.
The stock sits still at $100 throughout.
Only the days left move.

Days left	Delta	Gamma	Theta / day	Vega	Rho
45	+37.1	-1.79	$+1.21	$+8.67	$+18.38
21	+47.4	-2.52	$+1.73	$+11.88	$+17.35
7	+62.2	-1.84	$+0.99	$+15.57	$+16.36

Now move the stock to $104.00

Same three dates, same diagonal.
Now with the stock up at the call you sold — where the position stops improving.

Days left	Delta	Gamma	Theta / day	Vega	Rho
45	+28.3	-2.49	$+2.09	$+4.21	$+19.37
21	+32.8	-4.37	$+3.78	$+6.52	$+18.77
7	+39.6	-8.70	$+7.72	$+9.33	$+18.30

Gamma goes from -2.49 to -8.70.
Theta goes from $+2.09 a day to $+7.72.
Both climb, and neither is available without the other.
Delta now moves roughly 3 times as far for every dollar the stock travels.

Which is what DTE actually sets

The near leg sets the income and the sensitivity, exactly as it does everywhere else.
The far leg sets what survives, and it does not decay at anything like the same rate.
That difference is the entire structure. Take it away and you have a vertical.

The trap

The trap is what happens on a fast move up.

You are short a call and you do not own the shares. What you own is another option.

If the short call is assigned, you have to deliver shares.
You do not have shares. You have a long call, and exercising it takes cash you may not have set aside.
The maths usually works out. The mechanics can still leave you short stock for a day, at a size you never chose.
It is the same problem a spread has on assignment, made worse by the two legs expiring on different days.

Why it matters

A diagonal is often recommended as a cheaper covered call. It is cheaper, and it is not the same trade.
The difference shows up exactly when the trade is working best — a strong rally — which is the worst time to discover it.

How this shows up in our tests — pending. No result has been published on this structure yet.