Boundary divisors -- irreducible components of the boundary of the moduli space of stable n-pointed genus zero curves

Let $\Delta_I$ 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 $I^c$ lie on the second component. Then $\Delta_I$ is an irreducible effective divisor.

Let $\delta_I$ be the class of $\Delta_I$. Then the classes $\{ \delta_I : \#I \geq 2, \#I \leq n/2, 1 \in I if \#I=n/2\}$ span the Picard group of $\bar{M}_{0,n}$. The relations between these classes are called the Keel relations.