Single linkage. The distance between groups is defined as the distance
between the closest pair of objects, one from each group.
A drawback of this method is the so-called chaining phenomenon: clusters
may be forced together due to single elements being close to each other,
even though many of the elements in each cluster may be very distant to
each other.
Single linkage clustering is essentially the same as Kruskal's algorithm
for minimum spanning trees. However, in single linkage clustering, the
order in which clusters are formed is important, while for minimum spanning
trees what matters is the set of pairs of points that form distances chosen
by the algorithm.