Diagonal Spread

Core Thesis

A Diagonal Spread combines a calendar and a vertical: different expirations and different strikes. It is used to express directional bias with carry income from short near-dated optionality.

Structure (Bullish Call Diagonal)

• Long back-month call at lower strike $K_1$.
• Short front-month call at higher strike $K_2$.
• Net debit $D$ in most constructions.

PnL Logic

• If spot drifts toward $K_2$ by front expiry, short leg decays while long leg retains value.
• If spot collapses, position can lose much of debit.
• If spot rallies too fast above $K_2$ before front expiry, short-leg pressure can dominate near-term mark-to-market despite long-back-month protection.

Greek Profile

• Net positive delta (directional bias).
• Often positive theta around target zone from short front decay.
• Net long vega from longer-dated long leg.
• Path dependency is higher than pure verticals.

Design Rules

• Place $K_2$ near near-term price target.
• Select back-month long strike for desired delta anchor (often 0.55-0.75).
• Tenor separation should be wide enough to create clear decay differential.

Management Framework

• Primary decision point is front-leg expiry: close, roll short call, or convert.
• Defend early if spot accelerates through short strike and extrinsic collapses.
• Track assignment and ex-dividend risks for short calls.

Failure Modes

• Underestimating path risk in rapid trend moves.
• Over-rolling short leg and compounding transaction costs.
• Treating diagonal like a simple debit spread with fixed payoff geometry.

Practical Checklist

• Is your near-term target near short strike and medium-term bias still bullish?
• Is net debit acceptable if underlying trend fails quickly?
• Do you have explicit roll triggers for the short leg?