The most important sub-division of a zodiacal sign is the Navamsa.

When a sign is divided into nine equal parts each part is a Navamsa, measuring 
3 1/3 degrees of arc.

In Aries, Leo and Sagittaurus, navamsas are governed by the lords of the nine signs respectively from Aries. In Taurus, the counting continues from the lord of the 10th, and so on.

Generally, most of the astrologers base their predictions on the Rasi and Navamsa charts.

Navamsa Chart forms part of a horoscope. In this chart each Rasi is divided into 9 parts and the location of each planet in such sub-divisions is indicated.

The Navamsa chart is considered important particularly to study the marriage compatibility of the horoscopes of a boy and a girl.