TSB Primer - Margin Explained
A margin account is a loan account by a share trader with a broker which can be used for share trading. The funds available under the margin loan are determined by the broker based on the securities owned and provided by the trader, which act as collateral over the loan. The broker usually has the right to change the percentage of the value of each security it will allow towards further advances to the trader, and may consequently make a margin call if the balance available falls below the amount actually utilised. In any event, the broker will usually charge interest, and other fees, on the amount drawn on the margin account.

If the cash balance of a margin account is negative, the amount is owed to the broker, and usually attracts interest. I Margin buying refers to the buying of securities with cash borrowed from a broker, using the bought securities as collateral. This has the effect of magnifying any profit or loss made on the securities. The securities serve as collateral for the loan. The net value—the difference between the value of the securities and the loan—is initially equal to the amount of one's own cash used. This difference has to stay above a minimum margin requirement, the purpose of which is to protect the broker against a fall in the value of the securities to the point that the investor can no longer cover the loan.
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