Twitter TWTR this morning announced that Jack Dorsey, its co-founder and interim CEO, has become the company’s permanent CEO. Dorsey will remain CEO of Square, the payments startup he founded which is expected to go public soon. The news was made official on September 30, per an SEC filing.

As part of the announcement, the company said Dick Costolo, Twitter’s former CEO, stepped down from Twitter’s board. Likewise, Adam Bain, the company’s chief revenue officer and a promising candidate for the CEO role, was promoted to COO.

Dorsey was previously executive chairman of Twitter’s board; that role is now vacant and the company will be recruiting a new outside board member for that role.

Twitter’s stock traded up more than 2% before the market opened on the news.

On a conference call this morning, Twitter board member and search committee head Peter Currie said Twitter’s board did not plan for Dorsey to take the role. The board even released a statement over the summer which said CEO candidates must commit full-time to the role. In other words, someone who wasn’t already CEO of another company. That’s understandable — it’s extremely rare for any executive to hold two jobs, let alone the top job at two public companies. As reports of the news trickled out, Dorsey’s split role has raised lots of questions from investors and analysts. But the board clearly changed its view.

“We didn’t get to a different answer until we saw Jack at work for three months,” Currie said. He cited Dorsey’s leadership skills, effectiveness in recruiting, ideas for product innovation, and “execution cadence” while interim CEO as factors in getting comfortable with giving him the job. “Jack has been meeting and surpassing expectations while doing both,” he said, of Dorsey’s split CEO role.

“As the founder and inventor of the product, Jack has thought more about Twitter than anyone else,” Currie added. Dorsey was Twitter’s first CEO, but he was pushed out in 2008. He eventually returned to the company as executive chairman.

Currie noted that the appointment of Bain as COO would help Dorsey with “load balance” between his role as CEO of both Square and Twitter.

In the search process, Twitter held ten board meetings, 15 search committee meetings, and more than two dozen one-on-one meetings with candidates, Currie said.

Bain noted that Dorsey’s turn as interim CEO has had a “profound effect” on the company. “He’s got the senior staff working better together than at any point in our history,” he said. “There’s more cohesion and collaboration across the team now. He’s begun changing how work is done at the company.”

Twitter’s workforce also feels this way, according to Bain. “The employees are unanimous in their support of Jack, and also the vision he’s got for the company,” he said.

Dorsey, Costolo, and Twitter’s board tweeted their announcements this morning. Read them below:

Here’s Costolo’s take:

He warned Bain against making jokes, a reference to a famous joke he tweeted on his first day as COO, which turned out to come true.

And lastly, Bain:

Read more about Twitter’s CEO search:

Can Jack Dorsey be the CEO of two public companies?

Let’s remember why Jack Dorsey was fired as Twitter’s CEO

8 CEOs (and others) who served double duty

Where did Dick Costolo go wrong?

Twitter is officially adrift now, but who will rescue it?

You can follow Erin Griffith on Twitter at @eringriffith, and read all of her articles here.

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