Measured by market capitalization, it is second only to Exxon Mobil among U.S. companies
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%.
[Follow Philip Elmer-DeWitt on Twitter @philiped]