Instrument humidifiers are designed to fit inside your instrument, where it slowly releases moisture to create the proper humidity. (Although we suggest just placing it inside your case.) Regular use will protect your instrument from cracking and keep the pads form drying out. Includes a paper humidity indicator. This is our largest for the most humidity. Works great placing it in the case when maximum humidity is is important. If you wish to monitor the humidity we suggest using a gauge instead of the paper indicator. Try our dial or Planet Waves digital gauge for best results.Extra large for added humidity.