Trine, in full
Four signs is a third of the wheel, so each element's three signs sit at the corners of an equilateral triangle. Fire gives Aries, Leo and Sagittarius. Earth gives Taurus, Virgo and Capricorn. Air gives Gemini, Libra and Aquarius. Water gives Cancer, Scorpio and Pisces. That triangle is where the word triplicity comes from, and it explains why trine and element get used almost interchangeably in casual astrology writing.
Ptolemy puts the trine with the sextile among the harmonious aspects, against the quartile and the opposition as disharmonious, and later tradition kept the split. Two points in trine are held to reinforce each other without friction, since they share an element and therefore a basic disposition. They never share a modality, though. A trine always joins one cardinal, one fixed and one mutable sign, which the tradition treats as enough internal variation to stop the combination going flat.
The compatibility pages here give the trine 5 out of 5, the top score the angle table produces, with a note that the smoothest angle is also the one most likely to go unexamined. That caution is traditional rather than a hedge added afterwards. Astrologers writing about natal trines regularly point out that whatever arrives easily tends not to get worked at.
- Astrology
- Nothing else in common use
A worked example
Taurus, Virgo and Capricorn form the earth trine, occupying 30 to 60, 150 to 180 and 270 to 300 degrees of the wheel, each 120 degrees on from the last. A planet at 10 degrees of Taurus sits at 40 degrees of longitude and a planet at 10 degrees of Virgo at 160, so the separation is 160 - 40 = 120 degrees, an exact trine with no orb needed.
Sources
Ptolemy classes trine and sextile as harmonious aspects and quartile and opposition as disharmonious.
Ptolemy, Tetrabiblos I.13, "Of the Aspects of the Signs" (Loeb translation, LacusCurtius)
See also
Where it comes up on this site
Trine: frequently asked
The other two in your element. Leo is trine Aries and Sagittarius; Scorpio is trine Cancer and Pisces.
Three points each about 120 degrees from the other two, closing the whole triangle instead of one side. The corners land in a single element, so the pattern is named for it: a grand water trine, a grand fire trine.
Tradition calls it the easiest angle, which is not the same as the best outcome. The standing caution is that no pressure means no attention, so whatever a trine describes tends to get taken for granted. That reading is old, not a modern hedge.