WASHINGTON (AP) — As part of a wide-ranging effort to contain Wall Street's worst financial crisis since the Great Depression, the Securities and Exchange Commission took the unprecedented step Friday of banning short sales of stock in 799 financial companies. What follows are questions and answers about the government's decision:

Q. What is short-selling?

A. The activities of short-selling might sound lewd at times — there's naked shorting and covering your shorts — but the practice of selling stock short is pretty straightforward.

Investors sell short if they think the shares of a particular company are going to decline and they want to profit from the drop (see graphic on A10).

To do this, an investor borrows shares of Company X, usually from their broker, and then immediately sells them at their market price, say $100 per share.

If the share price falls, let's say to $80, the investor buys back the shares and returns them to the broker. The investor pockets the difference — in this case, $20 per share.

The practice can be risky. If the shares increase in value, the investor has to buy them back at a higher price, losing money in the process.

Q. Why did the SEC temporarily ban the practice?

A. The government and some money managers blame widespread short-selling by hedge funds for contributing to the collapse of Lehman Brothers Holdings Inc., American International Group Inc. and other troubled companies by driving down their share prices.

Shares of the two surviving investment banks, Goldman Sachs Group Inc. and Morgan Stanley, saw sharp price drops this week. On Wednesday, Morgan Stanley shares fell 24.2 percent while Goldman's dropped 13.9 percent.

Such sharp drops erode the market's confidence, which makes it harder for the companies to raise capital and could scare away clients, further weakening the companies.

The SEC's ban gives financial companies time to stabilize without the daily drumbeat of hedge funds shorting them on a coordinated basis, said Phil Orlando, chief equity market strategist for Federated Investors Inc., which manages $330 billion in assets.

New York Attorney General Andrew Cuomo said Friday his office will investigate whether some short sellers spread rumors and negative information to drive down the share prices of Lehman, AIG, Goldman and other firms.

Q. What's naked shorting?