The original UK Portsmouth Yardstick was developed by Stanley Milledge, who was in charge of handicapping racing at the Langstone Sailing Club in 1947 using the Island One design as the scratch boat (having a value 100).
[1] In the United States, the Thistle was chosen as primary yardstick for compilation in 1961 with a value of 83.0, which corresponded to its RYA PN rating at the time.
Other boats were compared using their DIYRA (Dixie Inland Yacht Racing Association) rating to produce the D-PN (Dixie-Portsmouth Number).
This proved successful and in 1973 the responsibility was passed from the DIYRA to the North American Yacht Racing Union.
Further extensions are being evaluated for offshore classes to take account of sail inventories, excess weight, etc.