Relative Positions of Matroid Algebras

A classification is given for regular positions D⊕D⊆D of Jones index 4 where -- EQUATION OMITTED -- is an even matroid algebra and where the individual summands have index 2. A similar classification is obtained for positions of direct sums of 2-symmetric algebras and, in the odd case, for the positions of sums of 2-symmetric C*-algebras in matroid C*-algebras. The approach relies on an analysis of intermediate non-self-adjoint operator algebras and the classifications are given in terms of K0 invariants, partial isometry homology, and scales in the composite invariant K0(−)⊕H1(−).