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

????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, 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%.