  A Lie G-torus of type X_r is a Lie algebra with two gradings -- one by an
abelian group G and the other by the root lattice of a finite irreducible root
system of type X_r. In this paper we construct a centreless Lie G-torus of type
BC_r, which we call a unitary Lie G-torus, as it is a special unitary Lie
algebra of a nondegenerate G-graded hermitian form of Witt index r over an
associative torus with involution. We prove a structure theorem for centreless
Lie G-tori of type BC_r, r \geq 3, that states that any such Lie torus is
bi-isomorphic to a unitary Lie G-torus, and we determine necessary and
sufficient conditions for two unitary Lie G-tori to be bi-isomorphic. The
motivation to investigate Lie G-tori came from the theory of extended affine
Lie algebras, which are natural generalizations of the affine and toroidal Lie
algebras. Every extended affine Lie algebra possesses an ideal which is a Lie
n-torus of type X_r for some irreducible root system X_r, where by an n-torus
we mean that the group G is a free abelian group of rank n for some n \geq 0.
The structure theorem above enables us to classify centreless Lie n-tori of
type BC_r, r \geq 3. We show that they are determined by pairs consisting of a
quadratic form K on an n-dimensional Z_2-vector space and of an orbit of the
orthogonal group of K. We use that result to construct extended affine Lie
algebras of type BC_r, r \geq 3. Our article completes a large project
involving many earlier papers and many authors to determine the centreless Lie
n-tori of all types.
