Where flat knots are not enough
Any rope cross or knot contains an even number of ends. Is there always an exception to a given rule? No... But very often, for each of them, there is a workaround...
Wouldn't this be nice if the number of ends in a knot could be an odd number? Yes it would... and it's impossible. But remember:
What do we want actually? We want to create easthetic bondages, that's it.
Now, what do we have? A bondage is nothing else but a beautiful woman beautifully surrounded by ropes, attached by knots. Its only limitation is our ability to tie knots. And this ability shall have no limit. Unfortunately our habits often blind us. They are stronger than invention. Let them not be...
What is our stronger habit relating to knots? WE TIE THEM IN 2 DIMENSIONS. Actually sometimes even 1D of which macrame is a good example. Therefore the whole knot and thus the VISIBLE part of it is tied in 2D. 3D knots are possible. Obviously they have an even number of ends. What advantage can you get from 3D knots? Well... 3 = 2 + 1.
A 3D knot does not need to be fully visible. One part can be 2D -and visible- and the other invsible. If you wish, you can split a 3D knot in a visible 2D part utilizing any number of ends, plus an invisible 1D part containing the compelementary unnecessary ends!