Left, Peter DaSilva for The New York Times; Jin Lee/Bloomberg News Evan Williams, left, and Jack Dorsey founded Twitter with Biz Stone in 2006.

As Twitter tries to manage its explosive growth, two of the company’s founders have traded roles.

Jack Dorsey, who conceived the idea for Twitter but was ousted as chief executive in 2008, said on Monday that he was returning to the company as head of product development. He has remained a Twitter director but has not been involved in daily operations.

Evan Williams, who had been overseeing products, has stepped back from working at Twitter on a daily basis, though he will remain a director, according to the company.

It is a crucial time for Twitter, which has faced criticism from users over the placement of ads and from software developers over its changing rules for building applications. With Mr. Dorsey, the company is returning to its roots.

“It was a tragedy for the period of two years when he wasn’t involved with the company that we were missing the founder,” said Peter Fenton, a board member and a partner at Benchmark Capital, an investor in Twitter. “Getting Jack back kind of completes the picture of getting us focused on the next phase of the company’s growth.”

Mr. Dorsey’s return is also a personal triumph. Until recently, he has spent little time at Twitter, and his relationship with Mr. Williams has been tense at times.

Mr. Dorsey started a new company, Square, which turns cellphones into credit card readers. He will remain chief executive there.

Mr. Williams, meanwhile, has been spending a sabbatical at Lake Tahoe in California, where he is considering a new start-up idea, according to people with knowledge of Twitter’s internal operations.

Twitter confirmed that Mr. Williams, who remains the largest shareholder, had been less involved for a couple of months but said that he continued to provide strategic advice. When he stepped down as chief executive last year and gave the role to Dick Costolo, a more business-minded executive, he said he was most passionate about building new products.

While Mr. Williams has been away from the office, Mr. Dorsey and Mr. Costolo forged a close relationship and suggested to the board that Mr. Dorsey return. Mr. Dorsey declined to comment, but privately he has said that he has regretted not being a part of Twitter’s recent growth and that Twitter has made some mistakes in designing the simplest product and not listening enough to what its users wanted.

People who have worked with Mr. Dorsey say he has an uncanny eye for technology product innovations. His role will also be to ensure that advertising does not diminish Twitter’s core product.

Mr. Dorsey’s title will be executive chairman. “Twitter’s three co-founders — Ev Williams, Biz Stone and Jack — have unselfishly played whatever role was most needed at the time to nurture the company and help the product reach its full potential,” the company said in a statement.

Mr. Dorsey will split his time between Square and Twitter, whose offices are walking distance from one another in San Francisco. Kay Luo, a spokeswoman for Square, said, “Square remains his top priority.”

Fittingly, Mr. Dorsey confirmed his new role, which The New York Times reported last week, in a post on Twitter on Monday. “Today I’m thrilled to get back to work at @Twitter leading product as executive chairman. And yes: leading @Square forevermore as C.E.O.,” he wrote.

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