This is the third of the five terms, or bounds, into which the Egyptian table divides Aries. It runs from 12 degrees of the sign to 20, a stretch of 8 degrees held by Mercury. It opens where Venus's term closes at 12 degrees, and it gives way to Mars's term at 20 degrees, so a planet moving forward through Aries leaves Venus's bound and enters Mercury's at 12 degrees exactly.
Set out in degree order, the five terms of Aries run Jupiter (0-6), Venus (6-12), Mercury (12-20), Mars (20-25) and Saturn (25-30). At 8 degrees this is the widest of the five, against a narrowest of 5 held by Mars and Saturn. The unevenness is not an accident of copying. No sign in the Egyptian table divides its five terms equally, and no two signs divide them the same way, which is why the table has to be consulted rather than derived.
The five rulers are the five planets visible to the naked eye. The sun and moon take no terms at all in this system, so each sign is shared out among Saturn, Jupiter, Mars, Venus and Mercury, every one of them holding exactly one stretch of it. A planet standing anywhere between 12 and 20 degrees of Aries is standing in Mercury's bound, and Hellenistic technique reads the lord of the bound from the degree occupied rather than from the sign as a whole. The table this atlas follows is the Egyptian one, transmitted by Vettius Valens in his Anthologies and recorded by Claudius Ptolemy in the Tetrabiblos, which stayed in ordinary use after Ptolemy set out his objections to it and offered boundaries of his own.