The Dow Jones Industrial Average is a price-weighted average of 30 large publicly traded companies traded on the New York Stock Exchange. The index was first calculated in 1885. It took 87 years to reach 1,000 and only took another 27 years to reach the 10,000 milestone. While the best-performing decade of the Dow was the 1990s, the best overall year was 1915, with the index up more than 80%. During the Great Depression, the index posted some of its worst years, including 1931, the worst year on record. During the current market rally, the Dow hit 20,000 on Jan. 25.