Summary Box: Apple is most valuable US co.

THE MILESTONE: Apple Inc. became the most valuable company in the United States, surpassing Exxon Mobil Corp.

HOW IT HAPPENED: Apple had briefly flirted with the top spot on Tuesday afternoon and kept it after the close of market on Wednesday. The stocks of both companies fell Wednesday, but Exxon’s lost more value, giving Apple the lead.

THE SIGNIFICANCE: It’s a substantial milestone for Apple, which has enjoyed a triumphant comeback since the 1990s, when it struggled to stay afloat before Steve Jobs returned to take the helm. It underscores Apple’s growth potential, while Exxon’s is far more limited.