A transversal design 
of order 
,
block size 
,
and index 
is a triple (
,
,
)
such that
1. 
is a set of 
elements,
2. 
is a partition of 
into 
classes, each of size 
(the "groups"),
3. 
is a collection of 
-subsets
of 
(the "blocks"), and
4. Every unordered pair of elements from 
is contained in either exactly
one group or in exactly 
blocks, but not both.
