This paper determines all the possible endomorphism algebras for polarizable
Q-Hodge structures of type (n,0,...,0,n). This generalizes the classification
of the possible endomorphism algebras of abelian varieties by Albert and
Shimura. As with abelian varieties, the most interesting feature of the
classification is that in certain cases, every Hodge structure on which a given
algebra acts must have extra endomorphisms.Comment: 19 page