There are generally two types of aligners: removable aligners and fixed aligners. Removable aligners: They have two distinct points: 1. The two points of contact with the tooth surface necessary to produce complex tooth movement are difficult to achieve in removable aligners (most removable aligners are intended for younger patients) Fixed aligners: by definition are aligners that are fixed to the teeth, and the vast majority of contemporary fixed aligners are evolved from the square arch aligner (the operating system is more cumbersome) The system evolved, and now most of us use straight wire orthodontic appliances (greatly simplifies the operation procedures of the previous square wire technology) There are also many branches of aligners: self-locking brackets orthodontic technology lingual orthodontic technology transmission straight wire orthodontic technology Invisalign invisible programmed orthodontic technology movable aligners (functional) fixed aligners