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Slack goes public today

The messaging service that has taken the corporate world by storm reaches a new milestone today: Its shares will begin trading on the N.Y.S.E. (Its ticker symbol: “WORK.”)

Slack could be valued at nearly $16 billion. The company set the “reference price” for its shares at $26, which puts its valuation at more than double the level of its last private funding round.

But the company is staging a direct listing, not an I.P.O. That means it isn’t using underwriters or actually determining the price at which it will sell its shares. (The only other major company to have held a direct listing was Spotify.)

So the $26 a share is a guidepost, and Slack’s stock could open much higher — or much lower, if investors are worried about issues like slowing growth.