Navamsa chart
The divisional chart classically read for marriage and the soul-level result of each graha. Each rashi is divided into nine 3°20′ navamsas; each navamsa maps to a rashi via the movable / fixed / dual rule.
Birth details
Reading the D9
Navamsa divides each rashi into nine 3°20′ slices, mapping each slice to a rashi via the classical rule: movable signs (Aries, Cancer, Libra, Capricorn) start at themselves; fixed signs (Taurus, Leo, Scorpio, Aquarius) at the 9th from themselves; dual signs (Gemini, Virgo, Sagittarius, Pisces) at the 5th from themselves.
The Navamsa is sign-based — house numbers are whole-sign relative to the navamsa lagna. A graha that is dignified in the rashi chart but afflicted in the navamsa is called vargottama-weak, and vice versa; the Navamsa adds a layer of nuance the D1 alone cannot resolve.
Because the navamsa lagna shifts every ~12 minutes, accurate birth time matters more here than for the D1. If your birth time is uncertain by 15+ minutes, treat the Navamsa as indicative rather than definitive.