Not an Iron Age design
The endless-looking knot is the best known image of “Celtic” art, but it is not an Iron Age one. The curving style of the Celtic Iron Age (La Tène art) relies on spirals, tendrils and comma shapes, not on strands that pass over and under one another. True interlace, in which bands or animal bodies are plaited together, became a major feature of art in Britain and Ireland from about the seventh century CE. It was produced by Christian monasteries and their stone-carving and metalworking workshops. Calling this tradition Celtic reflects the Irish and Irish-influenced monastic world that produced it, and the habit of the nineteenth-century revival that gave it that name.
Where interlace came from
Plaited patterns are very old and very widespread, which is natural for a design that echoes weaving, basketry and rope. Within the Roman world, braided and guilloche borders were standard in mosaic floors, including those of Roman public baths, and similar motifs appear in late Roman and Mediterranean art in metal, stone and manuscripts. The way ancient weaving produced over-and-under structures may have been one inspiration, but nobody can show that directly.
Interlace also appears in the art of Germanic peoples on the Continent and in England, often as animals whose bodies interlock. The Insular style seems to combine these sources. After the fall of Rome, contacts between Ireland, Britain and the Mediterranean world did not stop, and the likeliest view is that scribes and craftworkers learned interlace from imported objects, books and visitors, then adapted it.
Early examples in books and metal
The Book of Durrow, made in the later seventh century, has full pages of interlace and interlocked animals. By the eighth century, the Lindisfarne Gospels show a high point of the tradition, with intricate carpet pages in which birds and beasts are stretched and knotted into pattern. The Book of Kells, around 800, includes some of the densest knotwork ever produced. These books are the subject of the study of illuminated manuscripts, so here the interest is in the design itself.
Metalwork shows the same designs. Brooches, chalices and book shrines carry interlaced wire, cast panels and tiny animal bodies, all in a tradition that began before gospel illumination was fully developed and lasted after it.
Stone crosses
Large free-standing crosses, set up in monastic grounds in Ireland, Scotland, Wales, northern England and the Isle of Man, are among the main places where knotwork can still be seen in its original setting. Eighth-century Irish crosses such as those at Ahenny use bosses and plaited ornament. The tenth-century Muiredach’s Cross at Monasterboice combines panels of scripture scenes with patterns. Anglo-Saxon crosses, like those at Ruthwell and Bewcastle, often prefer vine scrolls with birds and animals, while crosses in the Isle of Man later show interlace blended with Scandinavian styles after the Viking Age settlement.
Pictish carvers also took up the same repertoire. Some Pictish symbol stones of eastern Scotland, especially those with Christian crosses on one face, have extensive interlace panels next to older Pictish symbols, showing how interlace could be placed beside quite different traditions.
The ring on the “Celtic cross”, the circle crossing the arms, is a Christian form with unclear origin. Popular stories say it joined the Christian cross with a sun symbol, but there is no document that tells us this, and the ring may simply have supported the arms of a heavy stone cross.
How the patterns were built
Interlace looks complicated, but many designs follow a simple method. A grid of dots is drawn, then lines are traced diagonally between them; breaks are put in at regular points and the strands are made to pass alternately over and under. The Scottish artist George Bain explained such methods in his 1951 book on Celtic art and showed how designs could be constructed from a plan, and his diagrams influenced many modern craftworkers. Early scribes and carvers did not necessarily work this way, because few working drawings survive, but a few unfinished pieces and trial sheets suggest planning with compass and straightedge.
This also means that “endless” is only partly true. Many patterns can be traced as a single unbroken line, but others consist of several separate loops, and medieval craftworkers did not always keep to one rule.
What knots may mean
A common modern claim is that Celtic knots symbolise eternity, the interconnection of all life, or the Trinity when three strands combine in a triquetra. These are attractive ideas, but they belong to later interpretation. No early medieval text that explains the meaning of interlace is known to scholars, and the same shapes were used by pagan Germanic craftsmen and, in other forms, by Islamic and Byzantine ones, who had very different beliefs. The simplest historical explanation is that interlace was popular because it was skilful, beautiful and flexible, and that in a Christian setting it was fitting for sacred things.
Some modern writers also claim that druids used knots for secret messages or protection. There is no evidence for that, since druids, as described in Roman sources, belong to the Iron Age, long before the first surviving interlace.
The modern knot
The twentieth-century growth of the knot as an emblem of Celtic identity owes much to the nineteenth-century revival, to books such as Bain’s, and to the jewellery, tattoo and tourist trades. That does not make it fake. Pieces in this style carry on a long craft, but it is a different thing from claiming that they preserve Iron Age religion. Seen in context, Celtic knotwork is a medieval Christian development with a long ancestry in the Mediterranean and northern Europe, and its real history is rich enough without the myths.