Mahadasha and antardasha
A mahadasha is the major period of one graha, and an antardasha is a sub-period of another graha inside it. Antardashas follow the same nine-graha order.
What it is
Within a mahadasha of N years, the antardasha of a graha with M years lasts N times M divided by 120. A 20-year Venus mahadasha has a Venus antardasha of about 3 years and 4 months, and a 6-year Sun antardasha of about 1 year.
How it is worked out or read
The sequence starts with the same graha as the mahadasha, then runs through the nine in the Vimshottari order. The timing of each is fixed by the arithmetic, and the tool shows the dates.
Where it appears on this site
Both levels are listed in the dasha tool. The how to read the timeline article shows an example.
How the antardashas are sized
A mahadasha is the major period of one graha, and inside it each graha has an antardasha, a sub-period in the same nine-graha order. The length of an antardasha is set by the mahadasha's years and the antardasha lord's years, using the formula of the mahadasha years multiplied by the lord's years, divided by 120. A twenty-year Venus mahadasha therefore has a Venus antardasha of about three years and four months, and a six-year Sun antardasha of about one year.
Because the years are proportional, each mahadasha contains its own complete set of antardashas, and their lengths add up exactly to the length of the mahadasha. The sequence starts with the lord of the mahadasha itself and then runs through the nine grahas in the Vimshottari order, so the reader can identify where a particular antardasha fits by its position in that order.
Reading a mahadasha and its antardashas together
Traditional readers look at the mahadasha as the main theme of a period and at the antardasha as its colouring. A Jupiter mahadasha with a Saturn antardasha, for example, is read as a period of expansion tempered by discipline, while a Jupiter mahadasha with a Jupiter antardasha is read as a period where the theme is strongly felt. The lord of the antardasha is read in relation to the mahadasha lord, and the two are considered together.
The relations between grahas, including their friendships and their houses, are used by many readers to judge how an antardasha sits within a mahadasha. The reader can look at the friendship table on the planetary friendship page to see whether the two grahas are friends, neutral or enemies, and whether that relation supports or strains the combined theme.
Using the timeline without overreach
Mahadashas and antardashas show when themes are active, and they can be a useful timing map. They should not be used as a single predictor of events. A period that looks difficult on paper may be quiet in practice, and the reader should compare the timeline with what is known about the person's life before drawing conclusions. The site shows each period and its dates so that the reader can check them against the chart and against other sources.