(Bloomberg) -- Slack Technologies Inc. opened at $38.50 on the New York Stock Exchange Thursday, valuing the office chat software maker at about $19.5 billion.

Shares started trading at 12:08 p.m., almost three hours after Slack Chief Executive Officer Stewart Butterfield rang the opening bell at the exchange.

Unlike a traditional initial public offering, Slack’s decision to pursue a direct listing means there was no offer price for the shares. Instead, advisers spent the morning gathering buy and sell orders to assess what the first-trade price should be. The stock exchange set a reference price Wednesday of $26 a share.

The market valuation of the company is based on the outstanding Class A and Class B shares, and doesn’t include restricted stock units.

Slack shares trade under the ticker WORK. Goldman Sachs Group Inc., Morgan Stanley and Allen & Co. advised on the listing.

To contact the reporter on this story: Elizabeth Fournier in New York at efournier5@bloomberg.net

To contact the editors responsible for this story: Aaron Kirchfeld at akirchfeld@bloomberg.net, Elizabeth Fournier, Matthew Monks

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