The system derives from the one that was used in the shipbuilding lofting process, with longitudinal axis labeled as "stations" (usually fuselage stations, frame stations, FS), transverse axis as "buttocks lines" (or butt lines, BL), and vertical axis as "waterlines" (WL).
Fuselage stations are traditionally nonnegative, thus the origin is located at the nose of the plane or, sometimes, ahead of it.
Two typical alignments for the base line are the tip of the nose (negative WL are possible) or the "nominal ground plane" (measurements will be nonnegative).
[4] Butt line ticks increase to the right of the pilot with the origin at the centerline.
When compared to the (right-handed) aeromechanics coordinate systems, the direction of the butt line is opposite to the y-axis.