The 3°20′ rule
A D1 rāśi covers 30° of sidereal longitude. D9 divides that sign into nine equal parts, each 3°20′. Count the divisions continuously from 0° Aries: the first part maps to D9 Aries, the next to D9 Taurus, and the signs keep cycling. The same rule can be stated by modality: movable signs begin with their own sign, fixed signs begin with the ninth sign from themselves, and dual signs begin with the fifth. A nakshatra pada is also 3°20′, so its boundaries line up with the 108 D9 divisions.
The calculator first calculates the D1 positions with Astronomy Engine 2.1.19 and this site's approximate Lahiri ayanamsha. It then maps each sidereal longitude to a D9 sign and a derived degree inside that sign. The D9 degree is a mathematical transform, not a new observed location in the sky. The South Indian diagram uses fixed sign cells, and the D9 Lagna marker comes from the transformed D1 ascendant.
Because a D9 division is small, an uncertain birth time or historical UTC offset can move the Lagna across a boundary. A different ayanamsha or ephemeris can also change a near-boundary graha label. This page requires a recorded time and place and flags transformed positions within 0.1° of a D9 boundary. It uses mean Rahu and Ketu and does not claim Swiss Ephemeris parity.
Compare D1 and D9 together
The D9 table lists each sidereal D1 sign and its derived D9 sign. A “same sign” marker shows a repeated D1/D9 sign, traditionally called vargottama; it is a coordinate label here, not a claim about a person's relationships or future. Start with the D1 rashi chart for the actual calculated sidereal positions, then use D9 as a separate derived view.
For the exact sign assignment rule, a worked example, and why sites can disagree, read the Navamsa calculation guide. Both pages state the ayanamsha and clock assumptions so two charts can be compared meaningfully.