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How do public-company buyouts work?
As a TL;DR version of JAGAnalyst's excellent answer: the buying company doesn't need every last share; all they need is to get 51% of the voting bloc to agree to the merger, and to vote that way at a shareholder meeting. Or, if they can get a supermajority (90% in the US), they don't even need a vote. Usually, a buying company's first option is a "friendly merger"; they approach the board of directors (or the direct owners of a private company) and make a "tender offer" to buy the company by purchasing their controlling interest. The board, if they find the offer attractive enough, will agree, and usually their support (or the outright sale of shares) will get the company the 51% they need. Failing the first option, the buying company's next strategy is to make the same tender offer on the open market. This must be a public declaration and there must be time for the market to absorb the news before the company can begin purchasing shares on the open market. The goal is to acquire 51% of the total shares in existence. Not 51% of market cap; that's the number (or value) of shares offered for public trading. You could buy 100% of Facebook's market cap and not be anywhere close to a majority holding (Zuckerberg himself owns 51% of the company, and other VCs still have closely-held shares not available for public trading). That means that a company that doesn't have 51% of its shares on the open market is pretty much un-buyable without getting at least some of those private shareholders to cash out. But, that's actually pretty rare; some of your larger multinationals may have as little as 10% of their equity in the hands of the upper management who would be trying to resist such a takeover. At this point, the company being bought is probably treating this as a "hostile takeover". They have options, such as: However, for companies that are at risk of a takeover, unless management still controls enough of the company that an overruling public stockholder decision would have to be unanimous, the shareholder voting body will often reject efforts to activate these measures, because the takeover is often viewed as a good thing for them; if the company's vulnerable, that's usually because it has under-performing profits (or losses), which depresses its stock prices, and the buying company will typically make a tender offer well above the current stock value. Should the buying company succeed in approving the merger, any "holdouts" who did not want the merger to occur and did not sell their stock are "squeezed out"; their shares are forcibly purchased at the tender price, or exchanged for equivalent stock in the buying company (nobody deals in paper certificates anymore, and as of the dissolution of the purchased company's AOI such certs would be worthless), and they either move forward as shareholders in the new company or take their cash and go home.