Regulation T (Reg T)
The Federal Reserve Board regulation that governs customer cash accounts and the amount of credit that brokerage firms and dealers may extend to customers for the purchase of securities.
Investopedia Commentary
According to Regulation T, you may borrow up to 50% of the purchase price of securities that can be purchased on margin. This is known as the initial margin.
Related Links
Margin Trading Tutorial
The Federal Reserve (the Fed) Tutorial
See also: Fed, Initial Margin, Margin, Regulation FD (Reg FD), Regulation G, Regulation U