The draft aft (stern) is measured in the perpendicular of the stern. The draft forward (bow) is measured in the perpendicular of the bow. The mean draft is obtained by calculating from the averaging of the stern and bow drafts, with correction for water level variation and value of the position of F[clarification needed] with respect to the average perpendicular.[citation needed] The trim of a ship is the difference between the forward and aft draft.