Signs and decans

The partitioning of skies - mystical home of various deities - to 36 smaller constellations was applied already by old Egyptians 2000 years A.D.; each constellation framed a so called decanate hour. Similarly observing these decanate constellations just before sunrise shows periodicity with different decan (or decanate) rising each 10 days. In the ancient Egypt the decan containing Sirius - fixed star rising just before sunbreak - announced beginning of annual spring floods of the Nile. This system was thus used as a celestial calendar.
Traditional astrology divides the sky into 12 signs each having three decanates. Here first decan in the sign emphasizes base character of the sign and the next two other decans taint their part of the sign with the character of two consequent signs of the same element. Having 12 signs we get 4 groups of three signs for each of the four basic elements. This is also called "triplicity". Decans are crucially important when considering position and strength of the planet as well as its possible interactions with other planets.
Signs of ancient Greeks slightly differed with the signs of new ages - e.g. Libra sign wasn't recognized and the corresponding part of sky with its stars was called Scorpion's pincers.
Since Ptolemaios and old Greeks a few thousands of years have passed and as the Earth's axis while retaining comparatively stable declination of about 23 degrees off the perpendicular to the plane of ecliptic shows yet slow motion on the sleek cone called precession the imaginative boundary of the first sign - Aries (the Ram) shifted two tens of degrees formally but in total deep into following Pisces. This motion accounts for average 1 degree shift per century and after all this the traditional zero of Zodiac now lies on the 6th or 7th degree of Pisces. This is taken into account seriously in Hindu astrology and their interpretation of each sign correspondingly differs. The offset value is named ayanamsa and is now roughly 24 degrees although there is no consensus on it really among Indian astrologers. The discrepancy between the 30 degrees wide conventional signs of Aries says something about the ecliptic itself not being quite stable over centuries and thousands of years otherwise the origin of Aries as compared to ptolemaian values around 285 AD would still be near the end of it and not in Pisces. Remember by fixing the origin of Zodiac we may put the anchor to some fixed star or to some historical moment and still every angle in the projection will always be in motion. Even the stars of constellation are in quite fast motion relative to each other and the shape of now visible constellations is a matter a few centuries of distortion. The ecliptic of Ptolemaios differed from that of our times like the boundaries of signs stated by modern astronomers will tilt significantly in the next centuries and will never more line up with world meridians and parallels as originally proposed. So if you hear about "ayanamsa" again don't let yourself being fooled. It speaks too simple geometry over history of distortion.