Ranked, unranked, and labelled trees

Until recently, most logical formalisms for trees dealt with ranked trees. In such trees, all nodes have the same fixed number of children (or, a bit more generally, the number of children of a node is determined by the label of that node). Over the past several years, however, the focus has shifted towards unranked trees, in which there are no restrictions on the number of children a node can have.

XML data is typically modelled as labelled unranked trees.