Investment Type: Debit
Strike Prices: OTM, both legs must have the same strike price
Expiration Month: current month for sold leg, later month for bought leg
Composition: Buy Put + Sell Put

Calendar spread is also called horizontal spread. Obviously, Calendar Put is identical with Calendar Call except that we change Calls into Puts and the strategy is identical. It’s whether we can achieve better yield using the Puts rather than Calls.

However with Calendar Put, the stock should not plunges too fast, otherwise we suffer loss even when our prediction is right. We buy longer term Put and Sell short term Put and both of them share the same strike price.

The best possible case is that if the stock’s price is at the strike price (of the sold leg) at the first expiration. While the worst will happen if the sold leg got assigned. When that happens, we have to buy the stock (which needs some more money), sell it back in the market and hope that the profit in the bought leg can offset the loss.