May one tie a scarf around his neck, or a tichel on ones hair?
It is best to do so with a bow over a knot. However there are Poskim which allow one to make even a double knot as long as the double knot is not tight. Other Poskim however forbid a double knot. In all cases, neither a double knot nor a bow knot, may be done with intent to undo the next day [past 24 hours]. Thus those women that do not undo their Tichel knots may not make any form of knot on their Tichel on Shabbos, even a bow knot.