  Let D_n be the dihedral group of order 2n where n \ge 2, 1 \to R \to F \to
D_n \to 1 be a free presentation of D_n. R^{ab}:=R/[R,R] becomes a
\bm{Z}[D_n]-lattice. We will study the module structure and the rationality
problem of R^{ab}.
