If you store your instrument in a dry area, simply immerse the Kyser® Lifeguard™ Humidifier in water, shake off excess, and then position it in the sound hole. It will preserve the wood on your instrument by keeping conditions at the proper humidity. The Acoustic Guitar Humidifier fits sound holes 3 3/4″ to 4 1/8”. The Classical Guitar Humidifier fits sound holes 3 3/8” to 3 3/4”. The Concert Ukulele Humidifier fits sound holes 1 22/25″ to 2 1/4″.