If you are part of a jam sessions with guitar or bass, you should know the key in which the others are playing. The pros among you may be able to hear that, but I either need the information from the chord sheet or someone knows the key ... As a programmer, I quickly have an app in mind simply recording a piece of a song and then recognizing the key via my iPhone. The result is the BlueKeyDetect app: One touch on the display plus a short piece of recording and the beats per minute with the key of the song are displayed. You should record short pieces several times or a slightly longer one to get the correct key displayed. There are limits, since the key in the song can also change and depending on the song, the recognition can be difficult anyway, but in practice it still works quite well if you take a few things into account: ① Find places in the song where there is a lot of sound. Drums and percussion don't do much to recognize the key. ② The longer you record the song, the greater the chances of recognizing the correct key. Alternatively, you can record short phrases several times and then choose the most detected key. ③ Background noise makes detection difficult, so if possible record only the song and not at a low level.