Let ΔI be the closure of the locus of curves with two irreducible components meeting at one node such that the marked points with labels in I lie on the first component, and the marked points with labels in Ic lie on the second component. Then ΔI is an irreducible effective divisor.
Let δI be the class of ΔI. Then the classes {δI : #I ≥2, #I ≤n/2, 1 ∈I if #I=n/2} span the Picard group of M0,n. The relations between these classes are called the Keel relations.