"When you buy on margin, there are key levels - as governed by the Federal Reserve Board's Regulation T - that must be maintained throughout the life of a trade. The minimum margin, which states that a broker can't extend any credit to accounts with less than $2,000 in cash (or securities) is the first requirement. Second, an initial margin of 50% is required for a trade to be entered. Third, the maintenance margin says that you must maintain equity of at least 25% or be hit with a margin call."
From http://www.investopedia.com/terms/m/minimummargin.asp:
"When you buy on margin, there are key levels - as governed by the Federal Reserve Board's Regulation T - that must be maintained throughout the life of a trade. The minimum margin, which states that a broker can't extend any credit to accounts with less than $2,000 in cash (or securities) is the first requirement. Second, an initial margin of 50% is required for a trade to be entered. Third, the maintenance margin says that you must maintain equity of at least 25% or be hit with a margin call."