An extra margin charge for a position large relative to the market's liquidity, reflecting the cost of unwinding it in a hurry.
Standard margin assumes you can exit at roughly the current price. That assumption fails when your position is a meaningful share of daily volume or of a contract month's open-interest. Clearing houses and brokers therefore add a surcharge that scales with size.
The charge is one of the reasons a strategy that works at small scale stops working when the capital grows. It is also a defence against the slow-motion disasters where a single participant's position is larger than the market can absorb.
Example: a clearing house charges normal margin up to 5% of a contract's open interest, then adds 25% above that. A fund holding 8% of open interest in a deferred month pays roughly a quarter more margin on the excess portion than a small trader pays.
Original diagrams for the ideas on this page. Illustrative, not real market data.
Margin and leverage. A $5,000 deposit can control a $100,000 position, which is 20:1 leverage. Because the loss is measured on the full $100,000, a 2.5% move against you halves the deposit and brings a margin call, and a 5% move uses all of it.
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