In the less-than-three months since Apple first passed Wal-Mart to hold the third highest market capitalization among U.S. companies, Apple's stock price has continued to increase while second-place Microsoft has seen its shares drift downward. The combination of events has quickly closed the gap between the two companies, and today Apple finally surpassed Microsoft in market capitalization for the first time in 20 years, although second-by-second fluctuations currently see the two companies frequently swapping positions.

Market capitalization is a popular metric for assessing the size of a company as a representation of the investing public's notion of the company's worth. The figure is derived by simply multiplying the number of outstanding shares of the company's stock by the share price.

In the case of Apple, slightly over 900 million shares outstanding times over $250 per share yields a market capitalization of about $228 billion. Microsoft, meanwhile, has a share price of only slightly over $26.00 but over 8.75 billion shares outstanding, giving it a nearly equal market capitalization to Apple.

With Apple and Microsoft now neck-and-neck, only Exxon Mobil's $282 billion market capitalization remains significantly ahead of Apple's among U.S. companies. Exxon Mobil's shares have dropped significantly over the past month, bringing its market capitalization down from a high of over $325 billion.