The trinity knot is a three pointed, interlinked knot typical of art work often seen on Celtic crosses and in the Book of Kells. While this form of art was refined and perfected in the Christian era, it was probably influenced by the triskele and organic flowing symbols found in Megalithic pre Christian rock art. The number three is significant in spiritual and religious beliefs. Some scholars believe the Celtic Otherworld was perceived as three dimensional in nature. This parallels the almost universal perception of the threefold nature of divine realms categorised as Earth Sky and Sea or Underworld. Many Irish deities like the Morrigan, Brigid and Ériu Fodhla and Banbha form part of triple deity, three entities in one. This also parallels the Christian trinity of the Father Son and the Holy Spirit represented in Ireland by the Shamrock.