Your acoustic and classical guitars deserve to be protected.They are investments that require steps to ensure their value grows over time. The Kyser® Lifeguard™ Humidifier does just that. If you store your guitar in a dry area, simply immerse the Kyser® Lifeguard™ Humidifier in water, shake off excess, and then position it in the sound hole. It will preserve the wood on your guitar by keeping conditions at the proper humidity. The Acoustic Humidifier fits sound holes 3 3/4" to 4 1/8”. The Classical Humidifier fits sound holes 3 3/8” to 3 3/4”.