  Tree rearrangement operations typically induce a metric on the space of
phylogenetic trees. One important property of these metrics is the size of the
neighbourhood, that is, the number of trees exactly one operation from a given
tree. We present an expression for the size of the TBR (tree bisection and
reconnection) neighbourhood, thus answering a question first posed in [Annals
of Combinatorics, 5, 2001 1-15].
