Measured by market capitalization, it is second only to Exxon Mobil among U.S. companies



Sources: NYSE, NASDAQ

At the close of market Wednesday, Apple’s AAPL share price stood at $244.11, down $1.11 (0.45%) for the day. Microsoft MSFT , on the other hand, closed at $25.01, down $1.06 (4.07%).

Why is this significant? Because Apple’s market capitalization ($221.36 billion) just overtook Microsoft’s ($219.35) for the first time in 20 years, making it the second most valuable company in America, after Exxon Mobil XOM .



Market cap — the product of a company’s share price and the number of outstanding shares — is a somewhat arbitrary measure. But in this case it does reflect the two company’s shifting positions in the market.

Apple’s share price has climbed nearly 520% over the past five years. Microsoft’s is up 9.5%.

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[Follow Philip Elmer-DeWitt on Twitter @philiped]