Model hierarchy is a simple way to structure data relationships. Within a hierarchical database model, data records are organized in a tree-like manner. In this structure, parent records can have multiple child records, and every child record has a parent record above it. All records can be followed to a single root node. Thus, a hierarchical database model is a simple one-to-many relationship. Hierarchical data models were more common in early computer databases, but are still utilized in operating systems and some web services. Today, most databases use relational models, which employ more complex attributes and tables to determine data linkages.