I should add: a consequence of allowing more than one incoming edge is that a DAG can have more than one "root," like this: A B \ / C With trees,

I should add: a consequence of allowing more than one incoming edge is that a DAG can have more than one "root," like this: A B \ / C With trees, if you have two roots, you'll have two disjoint trees, because there's no way for their descendants to meet without some node having multiple parents.