Margin in finance

In finance, a margin is collateral that the holder of a position in securities, options, or futures contracts has to deposit to cover the credit risk of his counterparty (most often his broker).

A example to explain what is margin：
Jean buys a share in Universal Widgets SA for $100, using $20 of his own money, and $80 borrowed from his broker. The net value (share - loan) is $20. The broker wants a minimum margin requirement of $10.

Suppose the share goes down to $85. The net value is now only $5, and Jean will either have to sell the share or repay part of the loan (so that the net value of his position is again above $10).

premium margin example 1
An investor sells a call option, where the buyer has the right to buy 100 shares in Universal Widgets S.A. at 90¢. He receives an option premium of 14¢. The value of the option is 14¢, so this is the premium margin. The exchange has calculated, using historical prices, that the option value won't go above 17¢ the next day, with 99% certainty. Therefore, the additional margin requirement is set at 3¢, and the investor has to post at least 14¢ + 3¢ = 17¢ in his margin account as collateral.

additional margin requirement
example 2
Futures contracts on sweet crude oil closed the day at $65. The exchange sets the additional margin requirement at $2, which the holder of a long position pays as collateral in his margin account. A day later, the futures close at $66. The exchange now pays the profit of $1 in the mark-to-market to the holder. The margin account still holds only the $2.

Example 3
An investor is long 50 shares in Universal Widgets Ltd, trading at 120 pence (£1.20) each. The broker sets an additional margin requirement of 20 pence per share, so £10 for the total position. The current liquidating margin is currently £60 in favour of the investor. The minimum margin requirement is now -(!)£60 + £10 = -£50. In other words, the investor can run a deficit of £50 in his margin account and still fulfil his margin obligations. This is the same as saying he can borrow up to £50 from the broker.