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They spin out each of their products (Stack Overflow, Trello) as separate corporations so Fog Creek didn't own Trello.

Edit: I guess the spinning-out doesn't say anything about ownership (though the article does say Trello did a round of funding after the spin-out)

Stack was never owned by Fog Creek was it? I thought the only relation was that they shared Joel as a co-founder.

that was my understanding as well, although there has occasionally been stuff written that suggested more too.