Sorry, but it's probably just a pretty way to fill up an empty space.
It is difficult to discuss the development and symbolism of Celtic knotwork without providing a lengthy history of the Celts for context, yet this is precisely what I am attempting to do in order to relate what I know in the fewest words possible. Therefore, if you feel this discussion is missing something, you're probably right.
I encourage you to learn about Celtic history to fill in the gaps as well as for its own sake. You can find a list of staring places in the bibliography. Most of the information I'm presenting here is taken from those sources, especially Celtic Art by J.R. Allen.
Prior to the Christian influence on the Celts (about A.D. 450), the only known Celtic artwork consisted of geometrical patterns such as spirals, key patterns, and step patterns. It has been suggested that the Celts' religion prevented them from depicting the works of the creator, namely animals, plants, and humans. That is why their artwork is restricted to geometrical patterns.
Many of these patterns have similar or identical counterparts in early Christian manuscripts and artwork. There can be no doubt that the Christian Celtic artwork was strongly influenced by pagan Celtic sources. However, it is only in the artwork of the Christian era that we see knotwork. Besides knotwork, the Christian Celts also added human, plant, and animal forms to their decorations.
Plaitwork, which is a pattern of interwoven (but unknotted) cords, is the earliest form of knotwork. (For examples of plaits, see the eight basic knotwork forms.) Plaitwork is not unique to the Celts; examples are found in many cultures. By breaking the plait's cords and reattaching them, knotwork patterns can be derived. The first examples of this practice came in the early 700's A.D. in Italy. This is about the time that the Book of Lindisfarne, the earliest illuminated manuscript featuring knotwork patterns, appeared.
J. Romilly Allen has identified eight basic knots from which most Celtic knotwork patterns were derived. These knots appeared in repeating patterns that were used to fill borders and empty spaces in illuminated manuscripts, sculptures, and jewelry. The knots did not, generally, appear as isolated elements.
Therefore, it's my opinion that the Celts did not use knots as specific symbols. They did not have different knots to represent specific ideas or concepts. Knots were just nifty ways to fill a space. The symbolism of connectedness and continuity seem apparent from simply looking at knotwork patterns. This may have been an intended effect, but I've uncovered no evidence to suggest that knotwork patterns mean anything more than that.
This is likely to disappoint a great many people. Many visitors to my Web site ask if I have a list of knots and what they mean, or if I know of a knot that symbolizes a particular concept. I'm sorry, but my research indicates that the Celts who first drew knotwork patterns had no such meaning attached to their work.
Now, there have been hundreds of years since knotwork patterns were first invented. It's not impossible that meanings have been attached to certain patterns over time. From what I can tell, such meanings do exist, but as far as I know, they are very localized and relatively recent in origin.
Addendum: documented meanings I've found.
In Brigit's Feast (Vol. 2 No. 1, pp. 9, 11) Frank Mills writes...
The interlaced patterns with their unbroken lines symbolize humankind's pilgrimage, both as a quest to return to our divine source and our spiritual growth as we move along in the quest. The pattern is to be mentally unraveled, which, while occupying the mind with a repetitive task, creates a deeper concentration enabling us "to see." In this it is akin to the use of a mantra or rosary beads.
...though in a footnote he says...
It must be remembered that in our interpretation of Celtic art we cannot know the mind of the ancient Celts who developed these forms, thus the best we can do is to hopefully 'read between the lines' correctly and make some educated guesses.
Meaning of Knotwork
My Celtic Cross (popular!)
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