Scrum of Scrums. Work together with multiple Scrum teams.

What is the Scrum of Scrums? And how can it be implemented in large projects with multiple teams?

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An essential feature of Scrum is the suggested size of the development team: in order to communicate efficiently, a maximum of nine people should work together. But what about if an organization is large and multiple departments are developing a product? If 20, 50 or 100 people are working parallel and the participating people are not only distributed over offices and levels, but also over buildings or even countries? How can requirements be managed and communication between the teams ensured?