This property, called the ultrametric inequality, is both necessary and sufficient for the representation of a dissimilarity measure by an ultrametric tree [Johnson, 1967; Jardine & Sibson, 1971]. As noted in the previous section, however, the ultrametric inequality is very restrictive. It implies that for any three objects in S, two of the dissimilarities are equal and the third does not exceed them. Thus the dissimilarities among any three objects must form either an equilateral triangle or an isosceles triangle with a narrow base.