Long Calendar Spread with Calls Option Strategy

Time is the variable being traded: a near dated call is sold and a longer dated call at the same strike is bought for a net debit, and the position does best when the stock sits near that strike while the front contract decays faster than the back one.

Fifteen day volatility of the underlying is shown next to the quotes, since a calendar opened into elevated volatility behaves differently once it subsides. Both legs are priced from bid and ask, and the most the spread can lose is the debit paid.