Two-way cluster analysis dendrogram of 66 phage types and 16 typing phages. The matrix of shaded squares represents the phage type × typing phage matrix, while the dendrograms show the clustering. The dendrograms are scaled by Wishart ‘s (1969) objective function, expressed as the percentage of information remaining at each level of grouping (McCune and Grace, 2002). Each square represents the presence (black) and absence (white) of a reaction with a given typing phage. The three phage type clusters and the 4 typing phage clusters are indicated at the node with numbers.