Each year since the eighth century, Hindu pilgrims have traveled to one of the four sacred cities—Hardvar, Prayag, Ujjain, and Nasik— each located on a different sacred river—to seek forgiveness of sins as they bathe in the holy waters. According to Hindu mythology, the four cities became consecrated by the four drops of the nectar of immortality that fell upon them from the vessel that the gods used to carry the elixir of life away to heaven.
The ancient city of Prayag, now known as Allahabad, is a city of about 900,000 located on the Ganges River in southeast Uttar Pradesh in North India. Allahabad is called the Titharaja, "King of Tithras" (King of the Holy Cities), for it is located where three sacred rivers meet—the Ganges, the Yamuna, and the mythical Sarasvati, known as Sangam. (The Sarasvati, according to tradition, flowed from the Himalayas before it transferred its magical powers to the Ganges and disappeared into the north Indian desert.) The very act of bathing at the confluence (the Triveni) is believed to bestow a triple blessing upon the Hindu pilgrim.
"Allahabad." India Food and Fun.
[Online] http://www.indiafoodandfun.com/travel/allahabad.htm. Crim, Keith, ed. The Perennial Dictionary of World Religions. San Francisco: HarperSanFrancisco, 1989.