Measurements & numbers
Luff perpendicular (LP)
The shortest distance from the clew to the luff — the number behind "150% genoa".
LP is the length of the shortest line you can draw from the clew to the luff, meeting it at a right angle. On its own it is just a distance. Its use is as a ratio: LP divided by the boat's J measurement — the base of the foretriangle — expressed as a percentage.
That ratio is the name of the sail. A headsail whose LP is 1.5 × J is a 150% genoa. At 100% it is a working jib that fills the foretriangle and no more. At 135% you have the modern cruising compromise: real power, still tacks inside the shrouds on many boats.
This is why "genoa" and "jib" are not two different objects but two ends of one scale. See genoa vs jib.