Celtic knots are complete loops without any beginning or end, this unending style is called pure knots. The knots vary from simple to complicated ones. The use of only one thread highlights the Celts’ belief in the interconnectedness of life and eternity. They strongly believe in the symbolism reflected in Celtic knot works from the old Celtic myths and legends.

http://www.math.nus.edu.sg/aslaksen/gem-projects/maa/0203-2-11-The_Book_of_Celtic_Knots.pdf