Buttline: Although it sounds like a new form of country line dancing popular in Texas single's bars, it is actually a term used in aviation to refer to the location of objects in an aircraft along the right to left axis. It usually must prefaced with either right or left, as buttline 0 is the centerline of the aircraft. It is often abbreviated as RBL or LBL for right buttline or left buttline respectively. It is acceptable to omit a reference to right or left when referring to an object which is mirrored across the centerline on both side of the aircraft.
In reference to aircraft, right and left is always determined when looking forward in the aircraft.
Example: Since the windows are located at BL 52.25, attempting to locate a seat at RBL 58.00 is probably not going to work out so well, as that would be outside of the fuselage. (Tends to get a little breezy out there).