This is the fourth of the five terms, or bounds, into which the Egyptian table divides Sagittarius. It runs from 21 degrees of the sign to 26, a stretch of 5 degrees held by Saturn. It opens where Mercury's term closes at 21 degrees, and it gives way to Mars's term at 26 degrees, so a planet moving forward through Sagittarius leaves Mercury's bound and enters Saturn's at 21 degrees exactly.
Set out in degree order, the five terms of Sagittarius run Jupiter (0-12), Venus (12-17), Mercury (17-21), Saturn (21-26) and Mars (26-30). This term of 5 degrees sits between the two extremes of the sign, where the widest is Jupiter's at 12 and the narrowest are Mercury's and Mars's at 4 each. 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 21 and 26 degrees of Sagittarius is standing in Saturn'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.