Classical guitars with their light bracing are very susceptible to changes in humidity and this is one way to mitigate it. Easily installed and removed from the sound hole, Lifeguard Humidifiers regulate the humidity inside the instrument. The Lifeguard absorbs excess moisture in damp conditions, and humidifies in dry conditions. A smart, inexpensive way to protect the investment you've made in your guitars. Fits 3 3/8 - 3 3/4 inch sound hole.