You might hear the term "The Dow" a lot, but odds are, you're not entirely aware of which companies make up the Dow -- more formally known as The Dow Jones Industrial Average. Many people don't realize it, but the Dow is merely an average based on the stock prices of 30 companies. Yes, out of thousands and thousands of existing companies, just 30 make up the Dow. Here are a few:

  • AIG
  • Alcoa
  • Altria Group
  • American Express
  • Boeing
  • DuPont
  • ExxonMobil
  • General Motors
  • Merck
  • United Technologies

The list could change at any time, but it actually doesn't change all that often. (For the full list, click here.) The last time it changed was in April of 2004, when Bill Mann recapped the switcheroos. The list above may remain as is for years. (Learn about the math behind the Dow.)

You can invest in mutual funds or exchange-traded funds that are based on many different indices, such as the Dow. Learn more in our Mutual Fund area, and zero in on our index fund information there.

And though the list of the index's components is impressive, there are actually some good reasons to ignore the Dow. Learn why in "The Dow Is Useless."

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DuPont is an Income Investor selection, and Merck used to be one.

Longtime Fool contributor Selena Maranjian owns shares of no company listed in this article. The Fool has a disclosure policy.