When an investor borrows to buy more shares, using the shares themselves as collateral, he buys on what?

Buying on margin is borrowing money from a broker to purchase stock. You can think of it as a loan from your brokerage. Margin trading allows you to buy more stock than you'd be able to normally. To trade on margin, you need a margin account. This is different from a regular cash account, in which you trade using the money in the account. By law, your broker is required to obtain your signature to open a margin account. The margin account may be part of your standard account opening agreement or may be a completely separate agreement. An initial investment of at least $2,000 is required for a margin account, though some brokerages require more. This deposit is known as the minimum margin. Once the account is opened and operational, you can borrow up to 50% of the purchase price of a stock. This portion of the purchase price that you deposit is known as the initial margin. It's essential to know that you don't have to margin all the way up to 50%. You can borrow less, say 10% or 25%. Be aware that some brokerages require you to deposit more than 50% of the purchase price.

Borrowing money isn't without its costs. Regrettably, marginable securities in the account are collateral. You'll also have to pay the interest on your loan. The interest charges are applied to your account unless you decide to make payments. Over time, your debt level increases as interest charges accrue against you. As debt increases, the interest charges increase, and so on.