Here's how they decide who gets to join the Dow

Apple will replace AT&T on the Dow Jones Industrial index on March 19.

The index has 30 of the largest public companies in the US.

Via the S&P Dow Jones Indices website, here's the eligibility criteria:

"While stock selection is not governed by quantitative rules, a stock typically is added only if the company has an excellent reputation, demonstrates sustained growth and is of interest to a large number of investors. Maintaining adequate sector representation within the index is also a consideration in the selection process for the Dow Jones Industrial Average™. Companies should be incorporated and headquartered in the U.S. In addition, a plurality of revenues should be derived from the U.S."

The most valuable company in the world, with a market cap of around $750 billion, clearly checks all these boxes.

David M. Blitzer, Managing Director and Chairman of the Index Committee at S&P Dow Jones Indices, said in Friday's press release: "As the largest corporation in the world and a leader in technology, Apple is the clear choice for the Dow Jones Industrial Average, the most recognized stock market measure."

Congrats, Apple!