What is the idea behind a key in music?

Hmmm

There are two common staves.

Treble and Bass clefs. (google an image of them both).

A staff is the 'grid' for placing notes on it. Notes go on either a line or a space. The staff is written out with 5 lines and 4 spaces. (Count them from the bottom up).

Look back at the treble clef image. Notice the swirl that intersects the 2nd line (2nd line from the bottom).

This is significant; Treble clef swirling on the 2nd line denotes that line as G4. The treble clef is also called the G-clef.

Look at the Bass clef, also called the F-clef. The 4th line (4th line up, 2nd from the top) has a colon above below it ( ----:---- ) This line is F3.

Treble clef lines (from 1st line to 5th - bottom to top): E - G - B - D - F

Treble clef spaces (from 1st to 4th - the space above the 1st line to the space below the 5th line) - F - A - C - E

Bass clef lines (from 1st line to 5th, bottom to top) G B D F A

Bass clef spaces (from 1st to 4th, 1st space above bottom line to space below top line): A - C - E - G

The accidentals written at the beginning of the staff to denote the change for all measures does not tell you the key.

No flats or sharps: C Major, A minor, D dorian, E phrygian, F lydian, G mixolydian, B locrian, an atonal piece, a piece that's in a key but doesn't use the key signature notation. (Let this fly above your head, the point is that the flats/sharps do not necessarily tell you what key you're in).

However, if you have 1 flat on the treble clef 3rd line, that means that the B4 is now Bb (flat) 4. It does not change the note name,

Twinkle twinkle little star:

Treble clef.

F4 F4 C5 C5 D5 D5 C5

(F on first space, C on 3rd space, D on 4th line).

The thing that makes this melody iconic (if you hear it, you'd say "OH I KNOW THAT SONG, IT"S TWINKLE LITTLE STAR"), is the series of intervals (the distance between two notes)

F4 - F4 - C5 - C5 - D5 - D5 - C5

---PU--P5--PU--M2--PU---M2---

You could play the same melody starting on any series of notes.

Starting on E4: E4 E4 B4 B4 C#5 C#5 B4

Starting on D4: D4 D4 A4 A4 B4 B4 A4

etc.

The melody will have the same intervals and therefore sound similar, though 'shifted' up and down, called transposition.

Sometimes things are transposed up and down because of certain timbres of keys. (Beethoven had a fetish for C minor, for instance). Sometimes it's to deal with the range of a piece. Maybe the range is too high for a clarinet to play, so it's been lowered to fit in the clarinet range.

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This 'Now the treble clef rests on B' or whatever is wrong, blatantly wrong.

Treble clef remains constant. The first line (bottom) line of treble clef is always E4). If there is a sharp it's E#4, if there's a flat it's Eb4. If you're in Ab major it's Eb4, if you're in D major it's E4. If you're in G mixolydian it's E4.

There is one clef which shifts, which is the C-clef, it centers on each of the 5 lines, and is named differently based on where it is.

Soprano (bottom line), Mezzo soprano (2nd line), Alto (middle line), Tenor (4th line), Baritone (5th).

Each time the middle part of the clef on a particular line makes that line C4.

Let this fly over your head if it's confusing, because you're already confused and there's no need to try to absorb it if you're stressed.

Treble clef: does not 'shift' like you're talking about.

Thumb me down for the long post, but if you do anything go to musictheory.net

One thing about having a key or key signature is to avoid having a lot of sharps or flats in the music, but usually it is the composer's choice which key is used.

Your reach is far, far exceeding your grasp. You're making assumptions on very little knowledge - for instance your assumption that like 'in a key of C notes between the line represent A-C-E where as notes with the line going through them represent GBDF)' is completely wrong. That applies to music in *ALL KEYS*, but written in the bass clef.

You can determine the key by looking at the key signature, the sharps and flats - but even that is just the first, most basic step.

It's the key signature that's at the beginning of a piece of music and it indicates, basically, which notes need to be flattened or sharpened. The position of the treble clef doesn't change, whatever the key is. The lines and the spaces always represent the same notes (assuming the same clef is used). For example, the bottom line always represents E and the bottom space always represents F, but in some keys some notes need to be played a semitone (half step) sharp or flat.

I believe that the clarinet is a transposing instrument and maybe that's what's causing the confusion. I don't play clarinet so can't help there.

I don't know what key Axel F was originally written in or why it was written in a particular key - sorry.

Some keys suit a particular instrument better. D and G are popular “fiddle keys” for Irish and Scottish tunes. Instruments like tin whistle, some harmonicas and melodions are diatonic and can only really play in one key. A guitar is a chromatic instrument so ca; play in any key but normally someone arranging a tune for guitar wouldn't put it in Eb or Bb. If a singer is involved then a melody needs to be within their range and if they can't comfortably hit the top note in a song then it can be transposed into a sifferent key so that the note is lower.