It is well known that if every pair of treatments in a symmetrical balanced incomplete block design occurs in $\lambda$ blocks, then every two blocks of the design have $\lambda$ treatments in common. In this paper it will be shown that a somewhat similar property holds for symmetrical group divisible designs. In the course of the investigation there will be introduced certain matrices which are of intrinsic interest.
"Some Relations among the Blocks of Symmetrical Group Divisible Designs." Ann. Math. Statist. 23 (4) 602 - 609, December, 1952. https://doi.org/10.1214/aoms/1177729339