Keys and scales
Most music is in a particular key. If we say that a song is "in the key of C," this means that the pitch C sounds like the most stable “home note” (or tonic) for the song.
Likewise, most songs use notes within a particular scale — a collection of notes in order from low to high.
Here is the C major scale:
(The little dots show where these notes can be found on a piano keyboard.)
Most people tend to hear major scales as "happy" or "bright."
Make some patterns with the notes in the C major scale (the tonics are highlighted):
You can think of a scale as something like an artist's palette of paint. It's a subset of all available notes, chosen because of the way they relate to each other. Each type of scale has a distinct sound, and many people have particular emotional associations with particular scales.
You can build scales by adding more notes at specific distances (or intervals) above the tonic. The two most common types (or qualities) of scales are major and minor.