The quintile and biquintile are based on the division of a 360° circle by multiples of five. A quintile occurs between two planets that are 72° apart and the biquintile occurs between two planets that are 144° apart. If you keep dividing a 360° circle by multiples of five you will also find the vigintile (18°), decile (36°) and tredecile (108°).
From Aspects in Astrology by Sue Tompkins,
…a quintile will not only say something about an individual’s personal style but also the style and technical quality of their creative work. …The quintile series is strongly emphasized in the charts of people who are preoccupied with making, forming, linking and arranging things.
In some ways the quintile series has associations with Virgo and Mercury.
Being drawn to a particular way of linking information ideas together can indicate an obsession with a particular subject matter.
In Horoscope Symbols Robert Hand writes that the quintile series,
…seem to grant the ability to turn creative inspiration into concrete end products. …the quintile and the biquintile, the two most powerful aspects in this group, appear to be extremely so, perhaps more than all the three series aspects except for the trine.
The three series includes the sextile, trine, semisextile and quincunx. The obsessional quality of the quintile series also points to an association with Pluto. The ability to take disparate elements and combine them into a cohesive style shows a kind of translation, or even transformation, of information- all information- audio, visual, taste, etc…
Unlike other aspects, the quintile series doesn’t seem to be inherited. The quintiles are unique gifts and ways of linking information, ideas and concepts that are entirely your own. However, the presence or absence of quintiles doesn’t show whether a person will be creative, or whether that person will actually produce creative work.
Insight into the number five from tarot
The quintiles show the ability to connect information in a unique way. They imply a preoccupation with a way of connecting information. In the book Tarot Constellations: Patterns of Personal Destiny Mary K Greer writes that fives have specialized knowledge and the ability to communicate that knowledge in a particular style. Fives translate information and make it understandable to the general public. Fives are also concerned with teaching and learning. They absorb and pass on information and knowledge.
If you have a quintile series aspect, you may be actively learning about your quintile energy, or you may be actively teaching that energy to other people. Greer mentions that fives often become authorities on what they have learned and become teachers and guides on that understanding. Quintiles may point to particular energy qualities that you can use to guide other people.
The difference between quintiles and biquintiles
The quintile is a step between the sextile (60°) and the square (90°). The biquintile is a step between the sesquiquadrate (135°) and the quincunx (150°).
The quintile is wedged between the opportunity to develop a talent (sextile) and creative crisis (square) while the biquintile sits between uncomfortable dissonance (sesquiquadrate) and the need for adjustment (quincunx).
The biquintile seems to show a greater urgency to let go, or spread, an idea or style before changing it or moving on to something else. The quintile seems filled with more expectation and excitement, but less urgency.