NEW YORK — The S&P 500 closed at 7,749.44, a 0.27 percent gain from the prior session, adding 21.24 points on the day.

The index traces its origins to 1926, when the Standard Statistics Company launched the Composite Stock Index, a daily measure of 90 stocks. It succeeded the firm's 233-stock weekly index, introduced in 1923.

The early index posted continuous daily gains, rising from 17.66 in Dec. 1927 to 31.71 by Aug. 1929. The Wall Street crash of 1929 then sent it into a sustained decline, bottoming at 4.43 in June 1932. Despite a partial recovery to 18.09 in Feb. 1937, the index did not surpass its pre-Depression high until Sept. 1954, when it closed at 32.31.

In March 1957, the index expanded to 500 stocks and was renamed the S&P 500 Stock Composite Index. It crossed 50 for the first time in Sept. 1958 and nearly doubled to 94.06 by Feb. 9, 1966, lifted by the post-World War II economic expansion.

A bear market in the 1970s and weakness in the early 1980s stalled growth for more than a decade. The mid-1980s bull market then drove the index from 102.42 on Aug. 12, 1982, to 336.77 on Aug. 25, 1987.

On Oct. 19, 1987—Black Monday—the index lost 20.47 percent of its value, its largest single-day percentage decline on record, falling to 230.30 by Nov. 1987. It did not reclaim its pre-crash high of 336.77 until July 26, 1989.

The dot-com boom pushed the S&P 500 past 500 on March 24, 1995, past 800 on Feb. 12, 1997, and past 1,000 on Feb. 2, 1998. The index hit an intraday high of 1,552.87 on March 24, 2000.

The 2002 downturn drove it to 768.83 by Oct. 10, 2002. It did not surpass its March 2000 intraday high until Oct. 11, 2007, when a brief bull market produced a record closing high of 1,565.15 on Oct. 9 of that year.

The collapse of the U.S. housing bubble, the subprime mortgage crisis and the 2008 financial crisis triggered sharp volatility. The index closed up or down three percent or more 29 times in the fourth quarter of 2008, including an 11.6 percent single-day gain on Oct. 13, 2008—its largest daily percentage increase on record. By Nov. 20, 2008, the index had fallen more than 50 percent from its Oct. 2007 peak, reaching 752.44, its lowest close since March 1997. It ended 2008 with a 38.5 percent annual loss and continued falling into the first quarter of 2009.

Today's close at 7,749.44 reflects the index's recovery and sustained growth since those lows.