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Margin equity consists of cash plus the market value of marginable securities in the account. (us.etrade.com)
Interest is earned on your performance bond (if in a T - bill) because the exchanges view Treasury bills as marginable instruments. (investopedia.com)
You'll get this call when your equity falls below Vanguard Brokerage's house maintenance requirement, which is 35 % for most marginable securities. (investor.vanguard.com)