More than six hundred years ago Roger Bacon taught that "Grammatica una et eadem est secundum substantiam in omnibus linguis, licet accidentaliter varietur" ('Grammar is, in its essence, one and the same in all languages, even though it differs in superficial features', Jakobson 1963:209). Why did Bacon believe this? Essentially, because he believed that the fundamentals of grammar arise from fundamentals of human thought, which are shared by all people and by all languages. This is the time-honoured tradition of universal grammar, now largely displaced by Chomsky's structure-based conception of UG in which meaning plays no real part. In historical perspective, then, the NSM program can be seen as a return to the older tradition – but with an important difference, namely, detailed and rigorous analysis of natural languages. As indicated in the previous chapter, the thirty-year program of semantic research inaugurated in Wierzbicka (1972) has reached the point where it has become possible to articulate a detailed and concrete account of exactly what the unity of all grammars consists in; that is, to delineate where the line runs between what is constant and what is variable, what is essential and what is "accidental", what is universal and what is language-specific. The main purpose of this chapter is to describe the proposed model of universal grammar; i.e. the inherent syntactic properties of universal semantic primes. We will also establish some basic metalinguistic terminology, building on the firm conceptual foundation of semantic primes.