Hairpins are metal, bone, wooden, horn or plastic I- or U-shaped pieces used to hold in place and decorate hair. The oldest ones are as old as ancient Egypt and Mesopotamia. Surprisingly, they improve.
Traditional hair pins used by dancers to keep their knots in place are u-shaped pieces of wire with several kinks along the sides and sometimes drops of glue or plastic on the sharp ends. They are 2 to 6 inches long, almost invisible in a hairdo (they come in black, copper and dark gray), and cost $3 to $12 per dozen, depending on size (yes, for a piece of wire; everything in amateur dancing is overpriced). The larger the knot, the longer hair pins you need. This is true and tried design. Unfortunately, they tend to fall out of the knot, especially if the hair are straight and silky. My daughter lost couple of dozens of grey 6" long hairpins before we found a solution. This new and improved hairpins are made by Scunci and are sold in K-mart for about $5 for ten.
They are 3" long, made of fake horn colored plastic, not flexible, and instead of a sharp turn in the lower portion of u they are widened to form a circle, while sides are doubled to match the width of the circle for strength. The strand of hair fills the circle and gets caught. This simple change proved very efficient: hairpins don't fall out on their own, but they are smooth enough to remove easily. I suppose for a very large knot you still will need to add longer wire hairpins, or a hair net, but for waist-long straight, fine and smooth hair they work well. They are not as invisible as wire ones, but small reddish-brown semi-circles look both "rich" and natural, and they don't clash with any embellishments used in ballet . Surprising how it's still possible to improve a thing that was around for three thousand years.