Nestings of BIBDs with block size four

Abstract In a nesting of a balanced incomplete block design (or BIBD), we wish to add a point (the nested point ) to every block of a ‐BIBD in such a way that we end up with a partial ‐BIBD. In the case where the partial ‐BIBD is in fact a ‐BIBD, we have a perfect nesting . We show that a nesting is perfect if and only if . Perfect nestings were previously known to exist in the case of Steiner triple systems (i.e., ‐BIBDs) when , as well as for some symmetric BIBDs. Here we study nestings of ‐BIBDs, which are not perfect nestings. We prove that there is a nested ‐BIBD if and only if , . This is accomplished by a variety of direct and recursive constructions.