Eris threw the golden apple into a party (at which She had not been invited!) The apple said "For the Fairest!". Juno, Minerva and Venus all claimed it. Jupiter refused to arbite (being wise) but a young man, Paris agreed to be their judge. Each offered him appropriate bribes, but what won his heart was the promise of Venus to give him the most beautiful woman in the world as his wife.
That was Helen, sister to Castor and Pollux, who all the kings of Greece had agreed to defend. Hers was the face that launched a thousand ships.
From that comes a measure of beauty. A millihelen is beauty enough to launch one ship....