In Closure Table, we store every path in a tree, not only direct parent-child references, but also grandparent-grandchild, and every other path, no matter how long. We even store paths of length zero, which means a node is its own parent. So if A is a parent of B, and B is a parent of C and C is a parent of D, we need to store the following paths: A-A, A-B, A-C, A-D, B-B, B-C, B-D, C-C, C-D, D-D. This makes it easy to query for all descendants of A, or all ancestors of D, or many other common queries that are difficult if you store hierarchies according to textbook solutions. - Aaron Terry