This period runs eighteen years of the 120 that the Vimshottari cycle divides among the nine grahas, and a life enters it at birth if the Moon stood in one of its lord's three nakshatras, Ardra, Swati and Shatabhisha.
The order of the nine periods is fixed and does not vary: Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury, and then Ketu again. It is the same order in which the nine grahas own the twenty-seven nakshatras, running three times through the list, which is what ties the dasha sequence to the Moon's position at birth. The nakshatra the Moon occupied gives the period a life begins in, and the fraction of that nakshatra already traversed gives how much of the period has already elapsed.
Each major period is subdivided in the same order and in the same proportions. A mahadasha is divided into nine antardashas, each running for the same fraction of it that its lord's own period bears to the whole 120 years, and the subdivision can be carried further into pratyantardashas and beyond. That is how the system produces timing fine enough to be tested: a combination in a chart is expected to show during the period and subperiod of the planets that form it.
Rahu holds eighteen years, second only to Venus. Rahu and Ketu are computed points rather than bodies, the two nodes of the Moon's orbit, and their inclusion among the period lords is one of the things that distinguishes this system from the Hellenistic material with which it shares a good deal else.