The “#1 rule” reminds developers about the stability of the applications’ binary interface via which applications communicate with and configure the kernel. What follows is intended to familiarize readers with the concept of an ABI, describe why ABI stability matters, and discuss precisely what is included in Linux’s stable ABI. The ongoing growth and evolution of Linux necessitate changes to the ABI, some of which have been controversial.

Source: LXer – A 10-minute guide to the Linux ABI