Celtic Knots 

Celtic knots(shown above is the Triquetra) are a highly popular decorative element dating back to the late Roman era, and have been adopted into architecture, monuments, tattoos, artwork, pagan symbolism, Christian symbolism, and Islamic symbolism over the centuries.

There are endless types of Celtic knots. The size, shape, pattern, and intricacy is entirely up to the creator. However, all Celtic knots bear a tell-tale 
interlacing style, where elements of the knots cross over and under one another to provide depth to the image. The purpose is to create the illusion that the knot is woven of unbroken lines. Celtic knots have no clear beginning and no clear end. They are therefore, to many, a symbol of eternity.

Celtic knots can be circles, stars, diamonds, squares, and virtually any other shape that has room for angles and overlapping lines. 

Early Celtic artwork, before the influence of the Roman empire, also included spirals, steps, and keys. Interlacing knots from as early as the 3rd century AD exist in Roman architecture, particularly mosaics. It is clear that the more angular knotwork of the Roman aesthetic took root heavily in Celtic culture,
and today such designs are considered almost exclusively Celtic, despite their association with other cultures and religions.

Some of the most striking examples of Celtic knot art can be found in the 8th century Christian Book of Kells, or Leabhar Cheanannais in Irish Gaelic. It is a Latin illuminated manuscript of the New Testament's four Gospels, along with some reference materials. The Celtic knots in the manuscript adorn drawings of Christian figures, but also mythological beasts, mundane animals, and Christian symbols.

Because there are no rules for the shapes of Celtic knots, there is no clear connection between knots and religious or cultural significance. Celtic knots are primarily a work of human creativity and ingenuity. There are, however, similar knots that do have religious importance outside the Celtic world. For
example, the symbol of the endless knot is revered in Tibetan Buddhism, and represents time and eternity.

Another knot, the triquetra, has become very popular in the New Age and neo-paganism movements. It is composed of three interlaced triangles and represents all things threefold. In paganism, it is sometimes used to represent the Threefold Goddess (Maiden, Mother, and Crone). It too can be found
in the Book of Kells since in Christianity it can be used to symbolize the Trinity (Father, Son, and Holy Ghost), but its origins are likely ancient Germanic.

A branch of Celtic neo-pagans, Celtic Reconstructionist Pagans, use the triquetra knot to represent the trifold goddess Morrígan, who is three mythological beings in one. They may also use the triquetra to represent the triad of earth, water, and air – or land, sea, and sky.

One author, J. Romilly Allen, has arrived at the conclusion that there are eight basic patterns in Celtic knotwork, but archaeologists and other experts cannot substantiate his findings. They are merely observations.