See margin maintenance. This is the yellow flashing light equivalent in your brokerage account. Danger, danger, Will Robinson. If you have a brokerage account with a 50% margin maximum and you have $30,000 in equity in there - but you have borrowed $14,500 from yourself - you only have about 500 bucks of room in there to spare. If the Fed Chairman sneezes and the market goes down a percent or two, you will suffer a margin call and be forced to sell securities likely just at the wrong time - i.e. when the market is down. Kids, don't let this happen to you.