Here in Canada we have really dry winters and even your humidifier on your furnace can keep up without some help. 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 3/4- 4 1/8 inch sound hole.