The Dow Jones industrial average, an index of 30 U.S. blue-chip stocks, is the oldest barometer of the stock market. On Friday, it jumped above 17,000 for the first time in its 118-year history.

WHAT IT IS: The Dow is a group of 30 big corporations, nearly all of them household names, and its dips and jumps reflect changes in their share prices. Other indexes, such as the S&P 500, open their doors to many more companies, providing a better overall picture.

The Dow may not be the best measure, but the oldest index remains the best-known shorthand for the stock market.

BEGINNINGS: Charles Dow's original Dow Jones industrial average had 12 big businesses, including American Cotton Oil and National Lead. Dow first published his average on May 26, 1896.

SELECT GROUP: The number of companies in the Dow expanded to 20 in 1916 and to 30 in 1928. The number has remained the same since then, though the cast of characters changes every few years. General Electric is the only remaining original member.

BEST DAYS: The biggest jump was on Oct. 13, 2008, when it soared 936.42, or 11 percent, to close at 9,387.61. That followed the announcement of a European plan to bail out financial institutions.

Its biggest percentage jump was more than 15 percent when it reopened on March 15, 1933. President Franklin D. Roosevelt had shut down the banking system earlier that month. During the extended bank holiday, Congress passed a law to shore up the financial system and Roosevelt created the country's first insurance for bank deposits.

WORST DAYS: The biggest point drop came on Sept. 29, 2008, when the average lost 777.68, or 7 percent. That was the day Congress rejected a plan by the George W. Bush administration to bail out the financial industry. In percentage terms, the biggest drop was on Oct. 19, 1987, when it fell 508, or almost 23 percent, to close at 1,738.74. An overvalued stock market and expectations of rising interest rates combined with computerized trading to create "Black Monday."

OWNERS: The index is now calculated and published by S&P Dow Jones Indices, a joint venture that is majority-owned by publishing giant McGraw-Hill. CME Group and Dow Jones own smaller stakes.

WHAT MOVES: A $1 change in any Dow stock is equal to a move of 6.42 for the index. In other words, if one blue chip rose $1, and the 29 other companies sat still, the Dow would increase 6.42.

EQUAL WEIGHT: Most indexes account for a company's overall market value, which is found by multiplying the number of shares by the stock price. For the Dow, price is all that matters. So, a $1 rise in the price of AT&T will have the same impact on the index as a $1 gain for Nike, even though AT&T's value is worth more than two Nikes.