  We discuss a (10+2)D N=(1,1) superalgebra and its projections to M-theory,
type IIA and IIB algebras. From the complete classification of a second-rank
central term valued in the so(10,2) algebra, we find all possible BPS states
coming from this term. We show that, among them, there are two types of
1/2-susy BPS configurations; one corresponds to a super (2+2)-brane while
another one arises from a nilpotent element in so(10,2).
