Margin

証拠金。証券会社から借りる購買力で、損失がプレミアムを超え得る無担保オプションで必要となる。

Margin is the collateral your broker requires you to keep when a trade could cost you more than you paid up front. Buying a call or put has no margin because your loss is capped at the premium, but selling options is different. When you write a naked call or a cash-secured put, the broker locks up a slice of your account to cover the potential loss, and that amount moves as the underlying and volatility change.

A simple example: you sell a put with a $50 strike. Your broker might hold a few hundred dollars as margin, not the full $5,000 you would owe if assigned. That leverage is exactly why margin selling feels attractive, and exactly why it bites people.

The common mistake is treating buying power as free money and selling more contracts than you could ever afford to be assigned on. If the stock gaps against you, the broker issues a margin call and can close positions at the worst possible moment. Size your trades against the real obligation, not the small margin figure.

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