  A brief introduction into bimodules of $II_1$-factors is presented.
Furthermore a version of the following result due to M. Pimsner and S. Popa is
derived: Let $N=M_{-1}\subset M=M_0 \subset M_1 \subset M_2 \subset \ldots$
denote the Jones tower of a $II_1$-factor $N\subset M$ with finite index. Then
the factor obtained by the basic construction from the pair $N\subset M_{n-1}$
is equal to $M_{2n-1}$.
