In Bagchi (2010) main effect plans "orthogonal through the block factor" (POTB) have been constructed. The main advantages of a POTB are that (a) it may exist in a set up where an "usual" orthogonal main effect plan (OMEP) cannot exist and (b) the data analysis is nearly as simple as an OMEP.In the present paper we extend this idea and define the concept of orthogonality between a pair of factorial effects ( main effects or interactions)"through the block factor" in the context of a symmetrical experiment. We consider plans generated from an initial plan by adding runs. For such a plan we have derived necessary and sufficient conditions for a pair of effects to be orthogonal through the block factor in terms of the generators. We have also derived a sufficient condition on the generators so as to turn a pair of effects aliased in the initial plan separated in the final plan.The theory developed is illustrated with plans for experiments with three-level factors in situations where interactions between three or more factors are absent. We have constructed plans with blocks of size four and fewer runs than a resolution V plan estimating all main effects and all but at most one two-factor interactions.Let s be a prime power. Let us recall the terminology of the m-dimensional Euclidean geometry and a few other terms and notations required for a symmetric experiment. We shall mostly follow the notations of Bose (1947).Notation 2.1 (a) F will denote the Galois field of order s. 0 will denote the additive identity of F .(b) F m will denote the vector space of dimension m over F . We shall think of the vectors in F m as column vectors.
