As a graph, a tree is represented by a set of labelled nodes connected by edges. Since most statistics describing a neuron‘s branching relate to the root (e.g. branch order, which increases after each branch point on the way from the tree root to all terminal nodes) it makes sense to attribute a directionality to the edges and to define the root as the node with the index 1. All edges lead away from the root. That defines their directionality uniquely.

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