The Menzel SL902-4 is an individual adjuster, also known as a fine tuner, designed specifically for the violin's E string. This small mechanical accessory attaches to the tailpiece and allows you to precisely fine-tune the tension of the E string.
A fine tuner allows you to finely adjust the tension of the E string without having to manipulate the main peg, which greatly facilitates tuning, particularly for metal strings that are sensitive to small tension adjustments.