Quasi-symmetric 2-design

Definition
A quasi-symmetric 2-design is a design of type $$(1,2)$$ with constants $$l_1$$ and $$l_2$$ such that any two blocks intersect in either $$l_1$$ points or $$l_2$$ points. For example, a Steiner triple system is a quasi-symmetric 2-design with $$(l_1,l_2) = (0,1)$$.

Stronger properties

 * Symmetric 2-design
 * Steiner triple system