????It takes two

????The post-Gates era has actually begun already, for all practical purposes. And that is much to the relief of Gates himself. The Microsoft co-founder spent a full decade executing a painstaking succession plan. Mostly he did it by progressively passing business leadership of Microsoft to his college pal Ballmer, 52, who became president in 1998 and CEO in 2000. Four years ago Gates told Ballmer privately he wanted to leave, and then two years ago announced publicly he would do it this July. "I've been No. 2," Gates says of his role in recent years. "I haven't been the decision-maker on anything."

????Of course, he also had to find a replacement for himself as product master planner and technology strategist. He and Ballmer decided to split those jobs up. After Gates arranged to purchase technology soulmate Ray Ozzie's faltering startup Groove Networks in 2005, he quickly set about grooming 52-year-old Ozzie, best known as the father of Lotus Notes, to succeed him as Microsoft's chief software architect. The other half of his technical responsibilities went to longtime colleague Craig Mundie, 59, who oversees Microsoft's $8 billion in annual R&D and spearheads long-term technical strategy.

????But one thing is clear: There wouldn't be any post-Gates Microsoft, at least not anytime soon, were it not for Ballmer's willingness to stay around and mind the store. "Every conversation Bill has had with me about being able to transition from Microsoft is always in the vein of he couldn't be transitioning if Steve wasn't there," says Melinda Gates, Bill's wife. "You don't walk away from your life's work if it's not going well. He just could never do that."

????Ballmer's management style has matured in the eight years he's been CEO. "He used to be in everyone's shorts, in every detail," says marketing boss Mich Mathews. "But he has changed profoundly. He is a general manager now." Ballmer made a conscious decision to step back from day-to-day management and take a larger view as he realized that his partner Gates was no longer going to be there to strategize alongside him.

????Even though he never was a serious computer programmer, by all accounts Ballmer is just as good at math as Gates is. He lives and breathes data. "Steve has a computer in his head," says Bob Muglia, a 20-year company man who heads the Server and Tools division. Ballmer expects his subordinates to be adept in math as well. He distributes 11-by-17 sheets filled with numbers detailing the progress of various operations. The numerals are so small that executives use transparent magnifier rulers to see them. But there are never any columns showing percentage changes. Ballmer believes people ought to do that in their heads. It saves space on the paper for more numbers.

????Ballmer has spent the past few years surrounding himself with a seasoned group of lieutenants. Kevin Johnson, 47, a 16-year company veteran who previously ran worldwide sales, now oversees both Windows and online services. To replace Raikes, who is about to become CEO of the Gates Foundation, Ballmer recently hired Stephen Elop, 44, to run the $19 billion Business division, which centers on Microsoft Office. Elop was CEO of software maker Macromedia until he sold it to Adobe, and more recently No. 2 at Juniper Networks. Bob Muglia, 48, the Server division chief, oversees development of the complex software employed inside business infrastructures. And Robbie Bach, 46, another 20-year veteran, runs Entertainment and Devices, which includes the Xbox game system and software for mobile phones.