A first look at Dwadashamsha
Dwadashamsha divides each sign into twelve parts of 2°30′; their names follow zodiac order, beginning with the sign itself.
STEP 1
Find the size of each part
Divide a 30-degree sign by twelve: each part spans 2½ degrees, written 2°30′. The first begins at 0°, the second at 2°30′, and the third at 5°. Each exact boundary belongs to the following part. At 30°, the next sign begins.
STEP 2
Name the parts in order
The first part takes the sign's own name. In Aries (Mesha), the sequence begins Aries, Taurus (Vrishabha), Gemini (Mithuna), and continues through Pisces (Meena). In Taurus, it begins with Taurus and finishes with Aries. Start a fresh twelve-part count for each sign.
STEP 3
Look up the ruler
Each part has the same ruler as the sign whose name it carries. A part named Sagittarius (Dhanus), for example, has Jupiter as its ruler. First find the part and its name; then look up the corresponding ruler.
STEP 4
Work an example
Take 17° Gemini. Since 17 ÷ 2.5 = 6.8, six parts are complete and the point lies in part seven: from 15° to less than 17°30′. Count Gemini, Cancer (Kataka), Leo (Simha), Virgo (Kanya), Libra (Tula), Scorpio (Vrischika), Sagittarius. The seventh part is Sagittarius, ruled by Jupiter. Choose another degree and repeat the calculation, including the starting sign in your count.