The Boveda maintenance free 2-way humidity control system for fretted, bowed and wooden woodwind instruments. Boveda creates the perfect humidity balance for your wood instrument—whatever its surroundings call for. This clever system automatically maintains the humidity inside your case to a safe level. no need for extra humidity readers or gauges. Boveda automatically adds and absorbs moisture slowly and precisely using only pure water and natural salts. Extremely simple to use and requires no maintenance other than replacing the pouch once it reaches the end of its lifetime. Small kit for violin, viola, ukulele, mandolin, electric guitar and small woodwind instruments (i.e clarinet). Contains 1 x cloth Boveda holder and 2 x 49% 70g Boveda humidity control pouches. Large kit for cello, double bass, acoustic guitars and large woodwind instruments (i.e bassoon). Contains 2 x cloth Boveda holders and 4 x 49% 70g Boveda humidity control pouches. Refill pack contains 4 x Boveda humidity control pouches in re-sealable pouch. prevents. warping. cracking. shrinkage & expansion.