Celtic Knots

How is a celtic knot constructed?
See http://www.wallace.net/knots/howto/

How is it possible to draw a celtic knot so that the thickness of the 'rope' being knotted is exactly uniform?
(Hint: try working on 1cm squared paper. Will it help to remember Pythagoras?)

How many different celtic knots could be drawn on, say, a 4 x 6 grid?
Or a 6 x 8 grid?
Can you draw a Celtic Knot on a 7 x 5 grid?

Can you recreate these celtic knotwork designs accurately?

More resources are available from ATM including posters and postcards.