The oldest of the Arabic Parts is calculated from three things: the ascendant, the Sun and the Moon. Take away any one of them and there is no Part of Fortune.

In a day chart it is ascendant plus Moon minus Sun, and in a night chart the Sun and Moon swap. What it means has been argued about for two thousand years — body, livelihood, luck, the physical circumstances of a life — but the formula has stayed put, and it is one of the few places where the Moon is load-bearing in a technique that is not about the Moon.