The best-selling D'Addario Acoustic Guitar Soundhole Humidifier makes it easy to properly and safely humidify your guitar. The Humidifier hangs safely in the Soundhole suspended by the instrument's strings so it never touches the body of the instrument. The humidifier releases the moisture slowly and evenly, keeping your guitar perfectly humidified.What Is Two-Way Humidity Control?Two-way humidity control works by continually responding to the outside temperature and climate by adding or removing humidity as needed to maintain a relative humidity inside the Reed Vitalizer bag.