In Hinduism, Vishnu, the Lord of the Universe exists in sixteen expanding Kalas or digits of manifestation. That is, one digit of His manifestation in the vegetable kingdom, two in the animal, and from five to eight in the human, according as souls pass from the savage at one end of the scale to the highest evolved state at the other.
The Lord’s manifestation in His Avataras ranges from nine to sixteen digits or rays. The full or Purna Avataras are those in whom all sixteen rays are present. Lord Krishna was a Purna-Avatara, with sixteen rays, Lord Rama was an Avatara of fourteen rays.
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Cited from http://www.sivanandadlshq.org/religions/avatara.htm
