According the Drekkana or Decanate Theory, there are 36 decanates. Each Zodiacal Sign of 30 degrees is divided by 3 and we get 3 decanates per sign.
The first decanate is owned by the lord of that Sign
The second decanate is owned by the 5th lord from that Sign
The Third decanate is owned by the 9th lord from that Sign
For example, if Aries is the Sign, the first decanate is owned by Mars, the second the fifth lord from Aries, Sun and third decanate the ninth lord from Aries, Jupiter.
So Drekkanas are one of the Seven Vargas ( Sapta Varga ) division and is important.
If Mars is in Aries, say in the 13the degree of Aries, then the decanate ruler of Mars is the Sun, as the Sun is the fifth lord from Aries, as Mars is in the second decanate