This is a chime for doors and windows. It's a set of 2, but one sometimes has poor contact (I will ship it in a way that makes it easy to tell which one it is). So, please consider it as a bonus. Originally, it was fixed with the included double-sided tape, but I removed it when I no longer needed it, and the double-sided tape has also been removed. By the way, I purchased it about 9 years ago. I was using it until recently, so please understand that there is some aging (⋆ᴗ͈ˬᴗ͈)” Thank you for your understanding (⋆ᴗ͈ˬᴗ͈)”