Brandenburger and Nalebuff point out that business does not have to be a zero-sum game. In fact, business done well often enables more than one side to win. Complementors are those products and services that complement a business. As Microsoft designs better operating systems, Intel benefits from increased sales of personal computers. As Intel upgrades its processor to be faster and more powerful, Microsoft benefits by being able to create better and more powerful software.
Complementary relationships are easy to identify.
The confusing part is that, at any given time, your complementors can become your competitors while your competitors become your complementors. In fact, they can be both, simultaneously. Not only that, but your customers can become your complementors, your suppliers your competitors, your complementors your customers, and so on. The focus of Co-opetition isn't simply how to play the game, but how, by understanding and manipulating the elements, to change the game to your advantage. Much of the book is spent on analyzing the effects of companies who played the wrong game.