Ratio spread
Encyclopedia
Ratio-spread is a strategy in options trading
Options strategies
Options strategies can favor movements in the underlying that are bullish, bearish or neutral. In the case of neutral strategies, they can be further classified into those that are bullish on volatility and those that are bearish on volatility...

 that involves buying some number of options and selling a larger number of other options of the same underlying market and (usually) the same expiration date, but of a different strike price
Strike price
In options, the strike price is a key variable in a derivatives contract between two parties. Where the contract requires delivery of the underlying instrument, the trade will be at the strike price, regardless of the spot price of the underlying instrument at that time.Formally, the strike...

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Ideally, this strategy should be used when either A) the implied volatility of the options expiring in a particular month has recently moved sharply higher and is now beginning to decline, or B) the trader believes for whatever reason that the underlying market of the option(s) will move steadily in his favor during the life of the option. The trader will use call options in this strategy if he believes the underlying market will move steadily higher, and put options if he believes the market will move steadily lower.

In the case of call options, the trader will buy some number of options having striking price X and write (sell) a larger number of options having striking price Y, where Y is greater than X. In the case of put options, the trader will buy some number of options having striking price A, but write (sell) a larger number of options having striking price B, where B is less than A.

The "straight" ratio-spread describes this strategy if the trader buys and writes (sells) options having the same expiration. If, instead, the trader executes this strategy by buying options having expiration in one month but writing (selling) options having expiration in a different month, this is known as a ratio-diagonal trade.

As with all option spreads, the trader in a ratio-spread will strongly prefer to buy options having a distinctly lower implied volatility than the options he is writing (selling).
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