Boveda for Music 49% RH 2-Way Humidity Control Replacement for Use in Fabric Holder. Size 70 for Fretted and Bowed Wood Instruments. Prevents Cracking and Warping. 4 Units in Resealable Bag CREATES THE PERFECT HUMIDITY BALANCE your wood instrument—whatever its surroundings call for—with the global leader in 2-way humidity control MAINTENANCE-FREE HUMIDITY CONTROL FOR FRETTED, BOWED, WOODEN WOODWIND INSTRUMENTS: acoustic/electric guitars and basses; violins, violas, violoncellos, double basses; ukuleles, mandolins, banjos; bassoons, oboes, clarinets; and more REVENTS WARPING AND CRACKING—Boveda precisely achieves and maintains between 45% and 55% RH relative humidity (RH). Recommended for most 3- and 4-season climates. (High absorbency Boveda is available for high-humidity climates.) EASY TO USE! No more adding water to foam, hoses and sponges! Just slip Boveda into its protective leak-resistant fabric holders (sold separately), place in the case and keep the case shut whether the instrument is inside the case or not LASTS 2-6 MONTHS based on instrument case/time of year/climate. Replace when rigid. All-natural salts and pure water are sealed in a leakproof membrane. Zip close resealable bag to keep these unwrapped Boveda fresh for up to 2 years Unopened, Boveda can stay fresh for up to 2 years in its overwrap and sealed bags.