The after perpendicular is an imaginary line used for referencing longitudinal positions on a vessel. Combined with the forward perpendicular, the mid-distance between is one way to locate midships, and to define a meaningful length for trim to be considered over. It is common for the after draught marks to be placed at (or very near) the after perpendicular. The AP also helps designers break the vessel up into different segments, as classification societies define areas where the ships structure must differ based on distances from the AP (and FP).
The after Perpendicular, Aft Perp, or AP can lie in a few different places depending on the arrangement of the vessel. When there is a rudder post the AP is drawn where the after side of the rudder post intersects the DWL. The AP can be drawn through the centerline of the rudder stock, as is customary in merchant ships. For naval vessels or vessels which have azimuthing drives (and thus, no rudders) it is customary to have the AP situated where the after most part of the vessel contacts the DWL or load line draught (if they differ).
For the purpose of assigning load line, the after perpendicular is taken as the aft extent of a waterline at 85% of the hull depth. If the rudder stock is behind this point (as would be the case with a counter stern), the AP is instead taken through it's centreline.