| > Key is described as choosing a certain set of notes from all the available ones. Not exactly, take for example the Cmaj scale. You can write different melodies that "gravitate" around one of the 7 notes and that effect makes up for a particular mode of the scale of C.
Even if the notes are exactly the same you can easily hear the different flavor of each mode. The easiest way to hear the modes is to play continuously the Cmaj scale against a C drone (a long note), but each time starting from a different grade of the scale of Cmaj. Same notes, different feelings. It's unclear to me (and I think controversial even among musicians) if you should use the mode name as a qualifier for the key and usually people just say that a song is in the key of $note, but you can definitely hear the difference. In particular, it is (was?) common to modulate to the relative minor/major key to highlight a section of a song (e.g.: key changes from Cmaj to Amin, same notes). > Is there any popular music that belongs to multiple keys because they only use the notes that are shared between them? It's very common in jazz, a classic example: Giant steps by Coltrane. The song continuously modulates in major thirds and loops around three keys. > Or, is there anything that doesn't belong to any key because it uses more notes than are allowed by any key? Look up "serialism" and "atonal music". |