The cut property at work. (a) An undirected graph. (b) Set X has three edges, and is part of the MST T on the right. (c) If S = {A, B, C, D}, then one of the minimum-weight edges across the cut (S, V - S) is e = {D, E}. X ∪ {e} is part of MST T 0, shown on the right.