(Balanced-tree) A technique for organizing indexes. In order to keep access time to a minimum, B-tree stores the data keys in a balanced hierarchy that continually realigns itself as items are inserted and deleted. Thus, all nodes always have a similar number of keys. B+tree is a version of B-tree that maintains a hierarchy of indexes while also linking the data sequentially, providing fast direct access and fast sequential access. The IBM mainframe VSAM access method uses the B-tree method. See Btrfs and VSAM.