One division of the Dwadashamsha spans 2 degrees 30 minutes, twelve to a sign and 144 in the zodiac. The ascendant crosses that arc in about ten minutes.
Of the sixteen vargas this one has the simplest construction. The twelve divisions of a sign are assigned to the twelve signs in order beginning from the sign itself, so the whole zodiac is reproduced inside every sign at one twelfth scale. A planet in the first 2 degrees 30 minutes of Taurus is in the Taurus dwadashamsha, in the next in Gemini, and so on around the circle back to Aries.
That self-similarity is why the D-12 was among the six charts of the shadvarga from the beginning, and it carries through the saptavarga, dashavarga and shodashavarga as well. It is one of only six divisions present in all four classical groupings.
Its subject is the parents and, through them, ancestry. The tradition reads the fourth house from the D-12 ascendant for the mother and the ninth for the father, following the same house significations used in the birth chart, which is consistent with the chart being a scaled copy rather than a rearrangement.
A ten minute tolerance is tight enough that the D-12 rewards a birth time taken from a hospital record and punishes one recalled decades later.