Western astrology assumes that each sign of the same triplicity is 120 degrees apart, forming angles to one another called trines, which are each equivalent to the 360 degrees of the circle divided by three. Trines are considered to be very powerful and yet very comfortable, free-flowing connections. This suggests that the signs in each element are very comfortable and compatible with one another and tend to have many of the same qualities in common.
Method of organization 
In traditional astrology, the concept of triplicity embodied several factors concerning the four classical elements and were considered of particular importance. Besides the four classical elements, two other ways triplicity could be organized were by rulership type and by season. However, neither are given much attention by modern astrologers. However, astrology by season, in particular has been adopted by astrologers who practice in modern Neopaganism, Druidism and Wicca.
Classical elements 
In traditional Western astrology there are four triplicities based on the classical elements. Beginning with the first sign Aries which is a Fire sign, the next in line Taurus is Earth, then to Gemini which is Air, and finally to Cancer which is Water -- in Western astrology the sequence is always Fire, Earth, Air, & Water in that exact order. This cycle continues on twice more and ends with the twelfth and final astrological sign, Pisces. The elemental rulerships for the twelve astrological signs of the zodiac (according to Marcus Manilius) are summarised as follows:
- Fire — Aries; Leo; Sagittarius - hot, dry
- Earth — Taurus; Virgo; Capricorn - cold, dry
- Air — Gemini; Aquarius; Libra - hot, wet
- Water — Cancer; Scorpio; Pisces - cold, wet
In traditional astrology, each triplicity has several planetary rulers, which change with conditions of sect--that is, whether the chart is a day chart or a night chart.
Triplicity rulerships are a very important essential dignity--one of the several factors used by traditional astrologers to weigh the strength, effectiveness and integrity of each planet in a chart. Many Hellenistic astrologers (for example, Dorotheus of Sidon) considered triplicity rulership the most powerful and demonstrable of the several essential dignities of a planet.
Triplicty rulerships (using the "Dorothean system") are as follows:
|Triplicity||Day Ruler||Night Ruler||Participating Ruler|
|Fire (Aries, Leo, Sagittarius):||Sun||Jupiter||Saturn|
|Air (Gemini, Libra, Aquarius):||Saturn||Mercury||Jupiter|
|Earth (Taurus, Virgo, Capricorn):||Venus||Moon||Mars|
|Water (Cancer, Pisces, Scorpio):||Venus||Mars||Moon|
One way in which triplicty rulerships were used by earlier astrologers was to divide a person's life into three periods: early, middle, and late. The condition of the triplicity ruler that is in sect in the chart is evaluated when considering the tenor of the early part of life; the ruler out of sect is examined for the middle of life; and the last third of life is evaluated by looking at the condition of the participating triplicity ruler. "Participating" rulers were not used after the Hellenistic period.
In medieval systems of astrology, each essential dignity was given a different weight. Domicile rulers were given 5 points of weight; exaltation rulers were given 4 points; and triplicity rulers were assigned 3 points of weight. This gives some idea of how much power medieval astrologers accorded to each essential dignity.
Actual seasons 
Traditional astrology also organizes triplicities according to the actual season in which the zodiac rose. For example, Aries, Taurus, and Gemini appear during the spring season.
The triplicities of seasonal elements in ancient astrology were the following:
- Spring - Earth - Taurus, Capricorn, Virgo
- Summer - Fire - Leo, Aries, Sagittarius
- Autumn - Air - Aquarius, Libra, Gemini
- Winter - Water - Scorpio, Cancer, Pisces
- Dorotheus of Sidon, Carmen Astrologicum, tr. by David Pingree. Originally published in Teubner series (Leipzig, 1976). Re-published by Ascella Publications (London, 1993). Re-published by Astrology Classics (Bel Air, MD), 2005.
- Late Classical Astrology: Paulus Alexandrinus and Olympiodorus (with the Scholia of later Latin Commentators). [Translated by Dorian Gieseler Greenbaum.] Archive for the Retrieval of Historical Astrological Texts [ARHAT] , 2001. P.6. (This is a translation of Paulus' Introduction along with the Commentary by Olympiodorus and related Byzantine scholia.)
-  Claudius Ptolemy, Tetrabiblos online [tr. by Frank Egleston Robbins] in the Loeb Classical Library, 1 volume, Greek text and facing English translation: Harvard University Press, 1940. Pp. 79-83.
- William Lilly, Christian Astrology, Book 3: An Easie And Plaine Method Teaching How to Judge upon Nativities, 1647. 2nd ed., 1659. Re-published by Astrology Classics (Bel Air, Maryland), 2004; by Ascella Publications, ed. D. Houlding, London, 2000; and [in facsimile of 1647 edition] by Regulus Press, London, 1985. [orig.] P. 104.
- Crane, Joseph Crane, A Practical Guide to Traditional Astrology. ARHAT publications (Reston, VA). 1997, 2007. ISBN 978-0-9662266-1-4. Pp.6-10.
- Crane, p. 8.
- J. Lee Lehman, Essential Dignities. Whitford Press (West Chester, PA), 1989. ISBN 978-0-924608-03-2. Pp. 132-138.