  This paper develops a clustering algorithm for formations in team sports,
with a focus on football games. Our method first clusters formations into
several average formations: `442,' `4141,' `433,' `541,' and `343.' Then, each
average formation is further divided into more specific patterns in which the
configurations of players are slightly different. The latter step is based on
hierarchical clustering and the Delaunay method, which defines the formation of
a team as an adjacency matrix of Delaunay triangulation. A formation clustered
using our method is expressed in a form such as `442-C1'.
