This rule is the second example of an inhibiting planet in action. Although the three outer planets are never used as main significators, they can be involved in prohibition. These kind of prohibitions can in no way effect aspects or other rules involving the Moon and main significators. This is the traditional method of prohibition. Mercury at 4 Cancer applies to Mars by sextile at 9 degrees Taurus, but Venus at 8 degrees Leo will prohibit this aspect by first making a square to Mars.
Prohibition can also occur by any of the other major aspects.
Mercury at 19N17 applies by parallel to Venus at 24N19 but is prohibited by the Sun at 23N02 which perfects with Venus before Mercury perfects with Venus. The Sun at 10N18 applies by parallel to Saturn at 05N17 but is prohibited by Venus at 03S53 which perfects with the Sun before the Sun perfects with Saturn. The Sun at 15.11 Cancer applies by trine to Mars' antiscion point at 21.32 Pisces but before perfecting Venus at 10.19 Leo will sextile the Sun's antiscion point at 14.49 Gemini therefore prohibiting the Sun Mars perfection.