Per Eder, J., in Falkonera Shipping Co v Arcadia Energy Pte Ltd (The Falkonera)  EWHC 3678 (Comm) at paras 101-102:
101. At the outset, it is right to say that, … as a matter of definition, it was indeed impossible to obtain satisfactory headlines and sternlines. But this requires some explanation. In summary, as a matter of terminology, when a vessel is moored alongside a berth, a headline is a line running from the bow of the vessel to a mooring point forward of the vessel; a sternline runs from the stern of the vessel to a mooring point aft of the vessel. These are to be distinguished from breastlines, which run roughly perpendicular to the heading of the ship. They do not, therefore, extend forward beyond the foremost point of the vessel; or aft behind the hindmost part of the vessel. If an Aframax comes alongside a VLCC to perform an STS operation, then the mooring configuration may well involve headlines and sternlines. If the bow of the Aframax is moored to the bow of the VLCC then that mooring line will by definition be a headline: it will stretch forward from the bow of the Aframax to the bow of the VLCC. By the same token, a line from the stern of the Aframax to the stern of the VLCC will, by definition, be a sternline.
102. It is obvious that the same configuration is impossible between any two vessels of the same sizes (not just VLCCs). Because they are approximately the same length, any mooring line that runs from the bow of one vessel to the bow of the other will effectively be a breastline (ie perpendicular to the heading of both vessels); and a mooring line from the stern of one vessel to the stern of the other will also be a breastline.