Prolonged exposure to too-dry or too-damp climates can cause loss of resonance and shorten the life of your valuable guitars. Easily installed and removed from the sound hole, Lifeguard Humidifiers regulate the humidity inside the instrument. The Lifeguars 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