Without a doubt, the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) has cemented its place as the cornerstone of the global non-proliferation regime. Referring to the NPT as the cornerstone is one of the few things that NATO and ICAN agree on, albeit for different reasons. 

The Former UN Secretary-General, the late Kofi Annan, even went further to hail the NPT as a “true cornerstone of global security”, as has every UN Secretary-General after him. It is obvious that the NPT is of great significance to global security, but the notion of a ‘cornerstone’ is a rather specific characterisation that the NPT enjoys. This specificity begs the question: what really is a cornerstone?