What is a ligature?
Ligatures are used to clamp the reed to the mouthpiece. The reed must be able to vibrate but it must not vibrate loose. The reed should be pressed firmly against the table of the mouthpiece with the flat side of the reed against the mouthpiece.
Before ligatures were truly commercially available musicians used cord that was tightly wrapped around the reed and mouthpiece. The Estonian Iwan Müller developed the first metal ligature. Cord is still used by a number of clarinetists who play a German system clarinet.
Ligatures are usually made of fabric (leather, nylon, or a combination of these) or of metal. However, there are also ligatures made of mesh and plastic. For a ligature to function optimally the reed must be held firmly without a change in sound caused by the pressure of the ligature on the reed. For this reason contact rails are used on each reed. These can run vertically or horizontally or in some cases are not rails but small contact points.