B-trees
	If we relax the restriction that each node can have only one key, we can reduce the height of the tree.
A m-way search tree
(a)	is empty or
(b)	consists of a root containing j (1²j<m) keys, kj, and a set of sub-trees, Ti, (i = 0..j), such that
(i)	if k is a key in T0, then k ² k1
(ii)	if k is a key in Ti (0<i<j), then ki ² k ² ki+1
(iii)	if k is a key in Tj, then k > kj and
(iv)	all Ti are nonempty m-way search trees or all Ti are empty

A B-tree of order m is an m-way tree
(a)	all leaves are on the same level and
(b)	all nodes except for the root and the leaves have at least m/2  children and at most m children. The root has at least 2 children and at most m children.

A variation of the B-tree, known as a B+-tree considers all the keys in nodes except the leaves as dummies. All keys are duplicated in the leaves. This has the advantage that is all the leaves are linked together sequentially, the entire tree may be scanned without visiting the higher nodes at all.