Ligatures are designed to hold the reed securely on the table of the mouthpiece while allowing the tip of the reed to vibrate freely. There are many different design concepts and combinations, making categorization difficult.
The oldest, and still preferred type in Germany is the Blattschnur, a simple thread with which the reed is tied onto the mouthpiece. It is inexpensive and after some practice, fast and easy to use but less practical if you have to test and compare reeds or change clarinets quickly. To ensure a good grip, the mouthpiece should have special grooves and a burr on the top to prevent the reed from sliding. (Picture)
Metal ligatures hold the reed very firmly onto the mouthpiece. Some will press the reed onto the mouthpiece over its entire length, allowing only the free end of the reed to vibrate. (Picture) Avoid those that may have sharp edges that can cut into the reed or even the mouthpiece.
Others have smaller contact points or bars which allow the reed to vibrate over its entire length. (Pictures) Some advanced ligatures allow the holding points to be exchanged with different materials, enabling comparison and choice between the tonal variations they might produce.
The ligature mass as well as its plating can have an influence on the sound and projection. Some players believe that heavier ligatures, such as those with gold plating, will result in a bigger sound.
Scratching the mouthpiece can be prevented by using either a mouthpiece cushion or some tape between the ligature and the mouthpiece.
Rubber- and textile ligature
Ligatures made out of softer materials such as rubber or leather can result in a softer, less focused sound. They can wear out over time and will need to be replaced should they become overstretched.
Some mouthpieces have tapers (the outside diameter proportions) which are different than most standard mouthpieces. They require special ligatures or ligatures with a more flexible material.
Another important criteria is the number of screws a ligature has and the way they operate.
On ligatures with two screws, always tighten the screw nearest the mouthpiece tip first to prevent the reed from sliding. Some ligatures with one screw have a tendency to push the reed to one side as they are tightened if they are slightly asymmetrical.
Have a spare ligature screw in the case as a good precaution.
Better ligatures will have a turning thread bar which automatically adjusts its angle with reference to the ligature screw. (Picture Vandoren)
Ligatures that are overtightened can actually warp a mouthpiece, resulting in a table which is no longer perfectly flat. Overtightening can also result in the threads on the ligature becoming stripped. The screws should never be turned to their limit and the two halves of the ligature should not touch each other when fastened.
Ligatures should always come with a mouthpiece cap which should stay in place on its own.