  Let T be a weighted tree with n leaves. Let D_{i,j} be the distance between
the leaves i and j. Let D_{i,j,k}= (D_{i,j} + D_{j,k} +D_{i,k})/2. We will call
such numbers "triple weights" of the tree. In this paper, we give a
characterization, different from the previous ones, for sets indexed by
2-subsets of a $n$-set to be double weights of a tree. By using the same
ideas,we find also necessary and sufficient conditions for a set of real
numbers indexed by 3-subsets of an $n$-set to be the set of the triple weights
of a tree with $n$ leaves. Besides we propose a slight modification of
Saitou-Nei's Neighbour-Joining algorithm to reconstruct trees from the data
D_{i,j}.
