This paper explains (in Part A) Wittegnstein’s understanding of the ‘grammar’ of our (or any) language, tracing its origins in the Tractatus’s concept of logical syntax, and then examining the senses in which Wittegnstein, in his later work, viewed grammar as being ‘arbitrary’. Then, armed with this understanding, it moves on (in Part B) to the task of examining how, within the framework of a Wittegnsteinian view of language, we should understand the inescapable ‘compellingness’ of logical necessity – what Wittegnstein calls the “hardness of the logical must”. Whereas it is often thought that Wittegnstein’s views on the nature of the ‘grammar’ of our concpets leads him towards a vitiatingly conventionalist or anti-realist understanding of necessity, in which its logical ‘superhardness’ becomes problematic, this paper will argue that there is actually no such tension in Wittegnstein’s thought. In fact, it will be argued, an understanding of the ways in which our conceptual grammar is arbitrary casts a great deal of light on how it is that our concepts can nevertheless support a logically superhard, and normatively commanding, notion of necessity. In support of this view, I distinguish Wittegnstein’s views on necessity from the ‘classical’ conventionalism of the Vienna Circle, and from the radical conventionalism of Michael Dummett, and defend Wittegnstein’s view from a powerful recent attack from Quassim Cassam.