Regulation T & Margin Requirements
The Federal Reserve's Regulation T sets the initial margin requirement — currently 50% of a purchase — and payment deadlines; FINRA adds minimum maintenance margin of 25% of market value for long positions (30% for short).
Reg T payment is due within two business days after the settlement date; unmet, the firm freezes or liquidates. Marginable securities include exchange-listed stocks and bonds — new issues and most mutual funds cannot be bought on margin (though fund shares held 30 days may serve as collateral).