Celtic key or maze patterns are one of the earliest styles of Celtic art, dating back to prehistoric times. If I am remembering correctly, the designs composed of triangular units evolved a little later. With knot work and spirals, key patterns seem to me to be iconic of Celtic art.
This sort of key pattern is built either on a grid or a rotated grid. I made another screen cast video showing the basics of how that works:
This key square is a minimal finished key pattern. The idea is, essentially, what would happen if I took a unit of a typical running key pattern border and, terminated it in all directions after one unit.