rect.hclust {stats}  R Documentation 
Draws rectangles around the branches of a dendrogram highlighting the corresponding clusters. First the dendrogram is cut at a certain level, then a rectangle is drawn around selected branches.
rect.hclust(tree, k = NULL, which = NULL, x = NULL, h = NULL,
border = 2, cluster = NULL)
tree 
an object of the type produced by 
k , h 
Scalar. Cut the dendrogram such that either exactly

which , x 
A vector selecting the clusters around which a
rectangle should be drawn. 
border 
Vector with border colors for the rectangles. 
cluster 
Optional vector with cluster memberships as returned by

(Invisibly) returns a list where each element contains a vector of data points contained in the respective cluster.
require(graphics)
hca < hclust(dist(USArrests))
plot(hca)
rect.hclust(hca, k = 3, border = "red")
x < rect.hclust(hca, h = 50, which = c(2,7), border = 3:4)
x