Histogrammes of initial distances d_ini within the infalling galaxy for merger model 7.5deg100vel (left) and fly-by model 5deg200vel (right) in bins of 0.25 kpc width. All particles within the respective COAs with apocentres above 30 kpc are included. In the merger case, the particles of both populations initially had similar radial distances from the centre of the infalling disc galaxy. In the fly-by case, the particles on retrograde orbits had on average higher initial distances from the infalling disc galaxy’s centre. Thus, they originate more closer to the rim of the galaxy. There are particles on prograde orbits coming from a similar region, but in addition to those many come from further inside the infalling’s disc. Thus, on average the prograde material originates from a more central part of the infalling galaxy.