There are many variation of button knots. Many button knots are tied with a few stands of contrasting cords together. some can slide along a cord and make for unique closure (sample). Most not combinations have some sort of button knot in the arrangement. Tassels are often preempted by a button knot. And they make great buttons.
A. Twist: Even a lightly twisted yarn can be used for a bottom knot but must be well plied so it will not split during tying. The cord does not have to be balanced to tie into a good button knot.
B. Density: The cord should be reasonable dense or the knot will tighten into oblivion.
C. Fibers: Because the knot has so many uses, any fiber can be used to tie into this knot. The soft or fuzzy fibers will not show off the knot well. Be sure the fibers are more suitable for the majority of the knots chosen for the project.
D. Textures: The button knot will look very unbalanced if a heavy textured yarn is used. It will be necessary at times to use the button knot on textured yarns because of the design of the entire project. Manipulate the knot to the best of your ability to make it round.
E. Wheel Requirements: Use a wheel that will give appropriate diameter for the project. It should have a fast whorl and good drawl in tension.