NEW YORK (TheStreet) -- Wall Street's history is replete with panicked sell-offs, two of which were severe enough to earn the nickname "Black Monday" long before this week's slide turned the headline catchphrase into a trending hashtag on Twitter.

Those events, in 1929 and 1987, and the financial damage they wrought, prompted U.S. markets and regulators to employ 18th-Century philosopher Edmund Burke's advice about using a knowledge of the past to avoid repeating history's mistakes. They set up a variety of safeguards to cope with so-called market volatility, which in the lexicon of the financial markets, is often synonymous with large declines.

Those include the New York Stock Exchange's Rule 48, employed both Monday and Tuesday mornings, which is designed to smooth openings during periods of extreme fluctuation by lifting a requirement that designated market-makers provide advance indicators for stock prices that have changed significantly from the previous day's close. The limits range from 50 cents for stocks under $20 to $5 for stocks between $100 and $500 and a 1.5% variation for stocks above $500.

Under those guidelines, indicators would have been required for both General Electric  (GE) and Apple  (AAPL) on Monday, when the companies' stocks opened 7% and 10% below their Friday closing prices, respectively. An indicator would also have been required for Apple on Tuesday, when the stock opened at $111.11, or $7.99 higher than Monday's close.

Better-known, perhaps, are the so-called circuit-breaker provisions of Rule 80 that halt trading in specific stocks or across markets when drops exceed specified thresholds that may "exhaust market liquidity," according to the Securities and Exchange Commission. 

"By implementing a pause in trading, investors are given time to assimilate incoming information and the ability to make informed choices," the New York Stock Exchange, acquired by Intercontinental Exchange (ICE - Get Report) in 2013, says in its rules manual.

The circuit-breakers, which apply across securities and futures trading markets, call for halts of 15 minutes when the S&P 500 falls 7% from the previous day's close (a Level 1 decline) or 13% (a Level 2 decline), provided that the drops occur before 3:25 p.m. The New York Stock Exchange's normal trading hours are 9:30 a.m. to 4 p.m.

If the index drops 20%, the markets halt trading until the next day, according to the SEC. The circuit-breaker provisions, which have been tied to the S&P 500 since 2013, were previously linked to the 30-member Dow Jones Industrial Average and triggered at quarterly declines of 10%, 20% and 30%. 

The changes followed the so-called flash crash of May 2010, when large fluctuations didn't trip the circuit-breakers. The alterations were designed to make the rules "more meaningful and effective in today's high-speed electronic securities markets," the SEC said in a May 2012 order. 

Following are the answers to some frequently asked questions about market sell-offs: 

-- How big was the stock market crash in 1929? On Monday, Oct. 28, 1929, the Dow Jones Industrial average dropped nearly 13%, according to Federal Reserve archives. It fell another 12% the following day, often referred to as Black Tuesday, and by mid-November, it had lost almost half its value. "The slide continued through the summer of 1932, when the Dow closed at 41.22, its lowest value of the 20th century, 89% below its peak," the Fed said in records posted online by its Richmond branch. The Dow Jones index didn't return to its pre-crash highs until November 1954, well after the end of World War II.

-- What about the crash of 1987: That crash occurred on Monday, Oct. 19, 1987, and was later referred to as "Black Monday," like one of the days marked by the 1929 crash. It was the first contemporary global financial crisis, according to Fed archives, and sent the Dow Jones Industrial Average down 22.6% in a single trading session. The loss remains the largest one-day stock market decline in history, according to the Fed. By comparison, even though the Dow Jones dropped more than 1,000 points in intraday trading on Monday, that represented a decline of only 6.6%.

-- How often has the New York Stock Exchange halted trading? Historically, the NYSE has closed to limit the fallout of severe market drops, including during military actions or political uncertainty. It shut down for a week in 1865 after the assassination of President Abraham Lincoln, for 10 days in 1873 when a Philadelphia banking firm failed and for more than four months amid the outbreak of World War I. A so-called circuit-breaker rule triggered a halt 30 minutes before closing time in 1997, and the exchange shut down for four days after the 9/11 terrorist attacks.

-- When do U.S. markets halt trading? Equities and futures markets in the U.S. end trading for the remainder of the day when the S&P 500 drops 20% from the close of the previous trading day.

-- When do U.S. markets pause trading? Current regulations suspend trading for 15 minutes when the S&P falls 7% from the previous day's close (a Level 1 decline) and when it falls 13% (a Level 2 decline). The circuit-breakers are triggered only during trading between 9:30 a.m. and 3:25 p.m.