Also called a multiway tree, a B-tree is a fast data-indexing method that organizes the index into a multi-level set of nodes. Each node contains a sorted array of key values (the indexed data). Two important properties of a B-tree are that all nodes are at least half-full and that the tree is always balanced (that is, an identical number of nodes must be read in order to locate all keys at any given level in the tree). A well-organized B-tree will have only three or four levels.