ANCIENT USAGEThe triquetra has been found on runestones in Northern Europe and on early Germanic coins. It presumably had pagan religious meaning and it bears a resemblance to the Valknut, a symbol associated with Odin.
Celtic artThe triquetra is often found in Insular art, most notably metal work and in illuminated manuscripts like the Book of Kells. It is also found in similar artwork on Celtic crosses and slabs from the early Christian period. The fact that the triquetra rarely stood alone in medieval Celtic art has cast reasonable doubt on its use as a primary symbol of belief. In manuscripts it was used primarily as a space filler or ornament in much more complex compositions, and in knotwork panels it is a design motif integrated with other design elements. Celtic art lives on as both a living folk art tradition and through several revivals. This widely recognized knot has been used as a singular symbol for the past two centuries by Celtic Christians, Pagans and agnostics as a sign of special things and persons that are threefold.
Christian useThe symbol has been used by Christians as a sign of the Blessed Trinity (Father, Son and Holy Spirit), especially since the Celtic Revival of the 19th century. When modern designers began to display the triquetra as a stand-alone design, it recalled the three-leafed shamrock which was similarly offered as a Trinity symbol by Saint Patrick. Some have also suggested that the triquetra has a similarity to the Christian Ιχθυς symbol. The triquetra has been used extensively on Christian sculpture, vestments, book arts and stained glass. It has been used on the title page and binding of some editions of the New King James Version.
A very common representation of the symbol is with a circle that goes through the three interconnected loops of the Triquetra. The circle emphasizes the unity of the whole combination of the three elements. It is also said to symbolize God's love around the Holy Trinity.
Modern useIn contemporary Ireland, It is traditional for a man to give a loved one a trinket such as a necklace or ring signifying his affection towards her. It is believed to represent the three promises of a relationship such as to love, honour, and protect. It is common for the design of the Trinity Knot to be engraved on a wedding ring and attaches to a Claddagh ring(another Irish traditional ring given for friendship or engagement).
The Trinity Knot holds major significance in neopaganism in that it is believed to represent the three stages of the Triple Goddess (Maiden, Mother, and Crone).
Modern Pagans use the triquetra to symbolize a variety of concepts and mythological figures.
Germanic Neopagan groups who use the triquetra to symbolize their faith generally believe it is originally of Norse and Germanic origins. Due to its presence in insular Celtic art, Celtic Reconstructionist Pagans use the triquetra either to represent one of the various triplicities in their cosmology and theology (such as the tripartite division of the world into the realms of Land, Sea and Sky), or as a symbol of one of the specific triple Goddesses, for example, The Morrígan.
The symbol is also sometimes used by Neo-wiccans and some New Agers to symbolize either the Triple Goddess; or the Three Realms: Earth, sky and sea; or the interconnected parts of our existence (Mind, Body, and Soul).
Also the triquetra is a symbol of protection in the Neo-wiccan religion. It is also said to represent family; Father, Mother and Child. However, this is not a traditional or mainsteam Wiccan idea. Wicca is primarily duotheistic - worshiping both a feminine Goddess and a masculine God - and does not include the concept of a divine child except at the winter solstice, when the God is seen as reborn from the Goddess. Within Wicca, the triquetra is mainly used as a symbol of the Triple Goddess of the Moon and Fate.
It is difficult to date the exact origin of the Celtic triquetra, and whether it was first used in a Christian or pagan context; the distinctive interlace/knotwork artistic style did not fully develop until ca. the 7th century A.D., but the triquetra is the simplest possible knot.