In several branches of representation theory, the existence of Auslander-Reiten sequences has led to new structural insights. for example, in the module theory of artinian algebras [11, 6], in the theory of lattices over classical orders [18, 2] over a complete discrete valuation domainR, and for the corresponding derived categories [12, 17]. For anR-order ⋀ in a finite dimentional algebraAover the quotient fieldKofR, Auslander and Reiten [2, 5] have charaterized the non-projective indecomposable ⋀-latticeEfor which an Auslander-Reiten sequence (AR-sequence for short)L→H→Eexists as those ⋀-latticesA-moduleKEis projective.