  Let $T = ([n], E)$ be a tree and let $D = ( d(i,j) )_{i, j \le n}$ be the
distance matrix of $T$. Let $S\subseteq [n]$. We give the first combinatorial
proof for a formula to compute the principal minor of $D$ indexed by $S$,
namely $\det D[S]$. This generalizes work of Graham and Pollak, as well as more
recent works.
