This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Chris Brennan's survey of Hellenistic astrology traces the threefold quality classification, cardinal, fixed and mutable in its later English vocabulary, back to its earlier Greek terms, tropika for the signs marking a solstice or equinox turning point, sterea for the solid or fixed signs, and disoma for the two bodied or mutable signs, showing the scheme as part of the same systematic Hellenistic astrology that produced the house and polarity classifications alongside it.
That documented earlier vocabulary matters for a quality entry because the later Latin and English terms, cardinal, fixed and mutable, are translations of an older technical language rather than an independent invention, and Brennan's survey records where a later tradition's own reasoning for the grouping, most often the seasonal explanation still given today, matches or departs from the earliest Hellenistic statement of it.
An entry therefore uses Brennan's survey for the documented Hellenistic origin and technical vocabulary of the threefold quality scheme, alongside a later primary manual such as Lilly's own textbook for how that inherited classification was actually applied by a working astrologer of a specific later period.