The three 10° parts
Each 30° sidereal rāśi is divided into three equal 10° parts. In the Parāśari Drekkana used here, 0°–10° stays in the rāśi itself, 10°–20° maps to its fifth sign, and 20°–30° maps to its ninth sign. Count the original sign as one. At exactly 10° or 20°, the next part begins.
For example, 5° Gemini maps to Gemini, 19° Gemini to Libra, and 21° Gemini to Aquarius. The degree displayed in D3 stretches the position within its 10° third across 30°; it is a derived coordinate, not a second planetary observation.
The tool first calculates this site's D1 positions with Astronomy Engine 2.1.19 and an approximate Lahiri ayanamsha, then applies this sign rule. It includes seven classical grahas and mean Rahu and Ketu. The D3 Lagna comes from the transformed D1 ascendant. It requires a recorded birth time, place, and historical UTC offset.
Read D1 beside D3
The table retains the sidereal D1 sign and degree, identifies which 10° third contains each position, and shows the derived D3 sign. The South Indian diagram fixes the signs in place. Its Lagna marker locates the transformed ascendant; it does not add a second sky observation.
For the traditional wording, worked boundary examples, and reasons charts can disagree, read the Drekkana calculation guide. Start with the D1 rāśi chart to check the underlying sidereal positions.