Normally, the Dancing Nataraja stands on his right leg and throws across the left leg. According to the ThiruviLaiyaadal puraaNam, there was a Pandya king who was grief-striken because of this. He felt that Nataraja has been standing on his right leg for eons of time. He prayed ardently to Nataraja to shift his stance and change his posture. He wanted to Nataraja to stand on His left leg and life up and throw across His right leg.
Because of his prayers, Nataraja appeared and hearing the Pandya's plea, shifted his legs. He stood on his left leg and lifted his right leg and danced thus.
This is known as 'Kaal maaRi aadiya patalam' among the 64 ThiruviLaiyaadal's - 64 Divine Sports of SriChockanatha Peruman of Madurai