The Decennials (Dekannikoi) technique, transmitted in Vettius Valens' Anthology Book IV, is a Hellenistic time-lord system that assigns life periods to planetary rulers.
This calculation uses the original Decennials system as transmitted by Vettius Valens (2nd century CE) and Hephaistio (5th century CE). In L1 major periods, each planet receives an equal 129 months (10 years 9 months). The sequence is determined by the order of planets encountered in zodiacal order from the ecliptic longitude of the sect light in the natal chart.
About Decennials and Hellenistic 10-year time-lord periods
Decennials (Greek dekaeteridia) are a Hellenistic time-lord technique in which life is divided into successive periods of ten years and nine months ruled by the seven traditional planets in turn. The method is described most fully by Vettius Valens in Book V of the Anthology and by Hephaistio of Thebes in his Apotelesmatika, and was used as one of the standard predictive systems in the Greco-Roman world alongside annual profection and zodiacal releasing. Each decennial period is itself divided into seven sub-periods called "partial times" (merikoi chronoi), giving an overall cycle of about seventy-five years from the chosen starting planet.
Each decennial period lasts ten years and nine months (10 years 9 months, or 129 months), corresponding to the lesser period of the planet in the Hellenistic tradition. The seven planetary periods sum to seventy-five years and three months when taken in sequence. The starting planet is determined from the natal chart - typically beginning with the Lord of the Year of the first profection or with the Sun in day charts and the Moon in night charts according to Valens - and the remaining six planets follow in Chaldean order: Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon.
Each ten-year-nine-month decennial is subdivided into seven partial times, each ruled in turn by one of the seven planets in Chaplain order beginning with the major decennial lord itself. The lengths of the partial times are derived from each planet's lesser period and add up to the duration of the parent decennial. The combination of decennial lord and partial-time lord (for example "Mars within Saturn") determines the texture of the months in question and is the practical heart of decennial interpretation: a peaceful decennial can contain a stormy partial time and vice versa.
All three are time-lord techniques, but they operate on different principles. Zodiacal Releasing distributes uneven periods derived from the lesser years of each planetary domicile, releasing from the Lot of Spirit or Fortune and producing peak periods and "loosing of the bond" events. Firdaria assigns fixed but uneven periods to the seven planets in a sect-determined Chaldean order, totaling seventy-five years. Decennials assign equal ten-year-nine-month periods to the seven planets, also totaling about seventy-five years. Each technique was used in parallel by Hellenistic astrologers, with Vettius Valens explicitly recommending that the testimonies of all three be weighed together.
The principal exposition of the decennial method is found in Vettius Valens, Anthology, Book V, where Valens explains the assignment of the starting planet, the lengths of the partial times, and the procedure for layering decennials with annual profection and the lord of the year. Hephaistio of Thebes, writing in the early fifth century, summarizes and refines the same material in Book II of his Apotelesmatika. Both texts are now available in modern English in the translations of Mark Riley and Eduardo Gramaglia. The method effectively disappeared from the Persian-Arabic tradition in favor of the firdaria but has been revived in the contemporary traditional astrology revival.