The filiform needle has five parts: the tip, the body, the root where body and handle meet, the handle by which it is held, and the tail. Each matters in use. A fine tip reduces pain on insertion, the body determines how much manipulation the needle will bear, and the root is the weakest point and the one that most often breaks.
Gauge and length are selected by site and by purpose. Facial points take fine, short needles; thick muscle such as the gluteal region takes longer and somewhat heavier ones. The usual range runs from 0.16 to 0.38 mm in diameter, and from 13 to 100 mm in length.