Let's just start up this blog on Ordinary Language Philosophy (and everything vaguely related) in media res:
Usually the difference between ‘constitutive’ and ‘regulatory’ rules is said to be that constitutive rules define an actions or praxis, while regulatory rule tells us how an action or praxis is performed well. On a semantic level, this is however rather vague. Semantically speaking, a better criterion for differentiating between ‘constitutive’ and ‘regulatory’ rules is that constitutive rules can be construed as analytic, whereas regulatory cannot be construed so. Chess can be specified solely in terms of its constitutive rules and independently of its regulatory rules, while the regulatory rules of chess cannot even specified without reference to its constitutive rules.
A ‘convention’ on the other hand is a (semantically speaking) regulatory rule that is being treated ‘as if’ it is constitutive. The Geneva Convention of human rights and, in fact, the whole of law are conventional in this sense. In terms of analyticity, ‘murder’ is certainly not ‘illegal’, but there is a sense in which law simply defines ‘murder’ as ‘illegal’.
A murder can be specified and defined independently of its illegality just as check-mate can specified independently of the regulatory rules of chess. But from the point of view of law, there is however a sense in which ‘murder’ is ‘illegal’.
The modal status of conventions and constitutive rules are hence ‘alike’, but they are also different. In the case of ‘check-mate’, we are confronted with our ‘inability’ to conceive of it as anything but the kings capture, while in the case ‘murder’ we are confronted with our ‘refusal’ to conceive of it as anything but ‘illegal’.
In the case of the Geneva Convention of Human Rights, this ‘refusal’ is often questioned.
PS: It is funny that, given that I here speak in terms related to analytical philosophy, “treated as if” in relation to law seems incredibly vague, while if I had spoken the tongue of continental philosophy (I am thinking of Agamben on law) such seeming vagueness would be the whole point.