ChartBender Options Trading
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Position Composition Risk Profile @ Expiration
Shown here, both contracts are ATM. They could both be OTM or ITM.
Long 1 back-month call,
Short 1 front-month call

Both contracts have the same underlying and same strike price, but different expirations.

Time value of short front-month call will decay faster than time value of back-month call.
Max Loss:  Total cost of the spread, which is the cost of the long call minus the credit from the short call.
E.g., $2.50 - $1.50 = $1.00
Max Gain:  Depends on the value of the back-month call when the front-month option expires.

Greeks-at-a-Glance
P&L caused by movement in stock price
Movement away from strike generates losses; toward strike generates profits.
P&L corresponding to changes in implied volatility
Long calendar spreads benefit from rising IV.  Falling IV is a detriment.
Time Decay
C:RR ratio is explained in "Decision Making"
Positive C:RR unless time value of short leg is too minimal (due to being deep ITM, OTM or near expiry.)