The first layer of software to run in the Android stack is a customized Linux kernel. Most of the customizations take the form of feature enhancements or optimizations to help Android and Linux work together more efficiently. Originally, Google made a point of contributing code it developed, but some of the features were rejected by the mainline Linux kernel developers for inclusion in the standard Linux kernel. This meant that to keep its desired code customizations, Google had to create a fork of the Linux kernel, which is permissible due to the license under which the kernel is released. Chapter 21, “Kernel and Module Management,” gives an introduction to the Linux kernel.