Key

By the number of sharped or flated notes. Say a song is written in the key of C, the key of C has no sharp or flat notes. So a song written in this key would have no sharp or flat notes. This is a simplistic answer but it's the best I could do. I don't know if there is a lesson on this or not. Google up "the circle of fifths", that will get you started down the endless road of music theory. :wink:

What determines the key of a song is the pitch of its 'tonal centre'. When you listen to most songs you can hear that one note above all others has a special quality, a feeling of stabilty and repose. It's often referred to as the 'home' note because it feels like the most important note. It's almost always chosen as the last note of songs because it sounds natural to end the song on the note that gives that strong feeling of having arrived home. An example would be the last note of Happy birthday. Listen to the finality that that note imparts to the song. That is the songs tonal centre or key note and whatever note that is - that's the key. Any note can be chosen to act as the key note. It's not the note itself that has this special quality. It's the way that the other notes are made to relate to it that gives it this quality

When we name the key of a song though we also include the 'mode' of the song. So if we say a song is in the key of C major, then that note C is the tonal centre and the other notes correspond to the C major scale. If the notes correspond to the notes of the minor scale, then we say it's in the key of C minor.

Thanks for the answers