Upper: position-velocity diagram along the major axis of the edge-on galaxy NGC 891. The squares show the rotation curves derived separately for the approaching and the receding side. The contour levels are: 0.2, 0.45, 1.0, 2.0, 4.0, 10, 20, 40 mJy   beam^-1. Lower: position-velocity diagram along the major axis for the model galaxy. The model has been constructed for the symmetric part of the disk. 1 arcmin = 2.76  kpc.