Because of changes in historic measurement systems, it is difficult to make meaningful and accurate comparisons of ship sizes. Historically, gross register tonnage (GRT) was a measure of the internal volume of certain enclosed areas of a ship divided into "tons" equivalent to 100 cubic feet of space (about 2.83 m3). Gross tonnage (GT) is a comparatively new measure, adopted in 1982 to replace GRT. It is calculated based on "the moulded volume of all enclosed spaces of the ship", and is used to determine things such as a ship's manning regulations, safety rules, registration fees, and port dues. It is produced by a mathematical formula, and does not distinguish between mechanical and passenger spaces, and thus is not directly comparable to historic GRT measurements. Displacement, a measure of mass, is not commonly used for passenger vessels. While a high displacement can indicate better sea keeping abilities, gross tonnage is nowadays promoted as the most important measure of size for passenger vessels, as the ratio of gross tonnage per passenger – the Passenger/Space Ratio – gives a sense of the spaciousness of a ship, an important consideration in cruise liners where the onboard amenities are of high importance.