Guitars, with few exceptions, are made of wood. And wood moves as the relative humidity of the air around them changes. When the humidity is higher than the moisture content of the wood, it absorbs moisture from the air and swells slightly. When the humidity drops, the wood will lose moisture and shrink slightly. If not controlled, this swelling-shrinking can cause damage to your guitar.
A number of styles of humidifiers have been developed over the years to lessen the impact of this swelling-shrinking. Many are designed to deliver moisture, but a few can remove moisture also. Some fit inside your guitar body. Others fit inside your case. Still others control the humidity of an entire room.
Click below on the links for different humidifier systems, to help you choose the system that works best for your own needs.
More on the way, please check back often.
Remember, music makes the world a better place, so keep playing!