D'Addario Guitar Humidifier Packs - Two-Way Humidification System is a must-have for any guitar owner. This system maintains the optimal 45-50% relative humidity level within your instrument case, by adding or depleting moisture. Not only does it protect your guitar from the risk of damage caused by excess moisture, but it also prevents the formation of mold and mildew. The system includes two packs - one for the humidifier and one for the dehumidifier. The humidifier pack slowly releases moisture into the air, while the dehumidifier pack absorbs moisture from the air. Each pack lasts for up to four months, and there is no need to refill or recharge them. This system is easy to use - simply place the Packs in your guitar case and close the lid. The Packs will do the rest, automatically maintaining the correct humidity level. The D'Addario Guitar Humidifier Packs are a great way to protect your investment, and keep your guitar sounding its best.