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Buying puts without owning underlying
In the money puts and calls are subject to automatic execution at expiration. Each broker has its own rules and process for this.  For example, I am long a put. The strike is $100. The stock trades at the close, that final friday for $90. I am out to lunch that day. Figuratively, of course. I wake up Saturday and am short 100 shares. I can only be short in a margin account. And similarly, if I own calls, I either need the full value of the stock (i.e. 100*strike price) or a margin account.  I am going to repeat the key point. Each broker has its own process for auto execution. But, yes, you really don't want a deep in the money option to expire with no transaction.  On the flip side, you don't want to wake up Monday to find they were bought out by Apple for $150.