First month of learning piano, someone told me to "play in the key of G." I nodded and played random notes. Had no idea what they meant.
The problem with most theory explanations: they assume you already know things. They use terms to define other terms. It's circular.
Let me try differently. Starting from actual zero.
Notes are just sounds at specific pitches. Higher pitch = higher on the keyboard, further right. Lower pitch = lower on keyboard, further left. We gave these sounds names so we could talk about them.
The naming system: A, B, C, D, E, F, G. Seven letters. Then it repeats. The A above G is still called A, just higher. This repeating pattern is called an octave – from one note to the next note with the same name.
White keys are the natural notes. Black keys are sharps (#) and flats (b). Sharp means one key higher. Flat means one key lower. The black key between C and D is both C# and Db. Same sound, two names depending on context.
A scale is a specific pattern of notes that sound good together. The major scale pattern: whole step, whole step, half step, whole step, whole step, whole step, half step. A whole step skips one key. A half step is the very next key.
Start on C and follow that pattern: C (whole) D (whole) E (half) F (whole) G (whole) A (whole) B (half) C. All white keys. That's why C major is the "easy" key – no sharps or flats needed.
Start on G and follow the same pattern: G (whole) A (whole) B (half) C (whole) D (whole) E (whole) F# (half) G. See how you need F# instead of F? That's because the pattern demanded a whole step from E, and E to F is only a half step.
This is why different keys have different sharps and flats. The pattern stays the same, but which notes you land on changes based on where you start.
Now "key" makes sense. When a song is in the key of G major, it primarily uses notes from the G major scale: G, A, B, C, D, E, F#. The note G feels like "home" – the place where tension resolves.
Why does this matter practically? If you're learning a song in G major, you know most notes will be from that scale. You know F will usually be F#. You know what chords are likely to appear. It's a roadmap.
Minor scales exist too. Same concept, different pattern. The natural minor pattern: whole, half, whole, whole, half, whole, whole. A minor: A, B, C, D, E, F, G, A. All white keys, just starting on A instead of C.
Minor keys sound "sad" or "serious." Major keys sound "happy" or "bright." This is oversimplified but useful for beginners.
The practical application: when someone says "this song is in D major," you immediately know the sharps (F# and C#), the likely chords (D, G, A, Bm, Em), and where the music will feel resolved. This helps with learning songs by ear and understanding how chords work.
MusicTheory.net has interactive lessons on this if you want to hear the concepts as you learn them.