An alternating knot diagram is a knot diagram in which the crossings alternate between "over" and "under" as you travel along the knot. An alternating knot is a knot which has an alternating diagram.

For example, the diagram for the trefoil knot 31 below is alternating, whereas the diagram for the knot 819 is not alternating.

31 819


[1] Lickorish, W. B. R., An introduction to knot theory. New York: Springer (1997).