system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does it mean to sing a song in a particular key?
gpt
**First and Foremost: The vast majority of people saying this have no idea what it means! Especially Singers.**

Notes are Frequencies. Doubling a Frequency results in a Note that sounds "kinda the same but higher", which is why it is treated as a higher version of a lower note. Example: 440Hz is an A, but so is 880, 1760 and even 220 and 110 or 55. The same Note but higher or lower is called an Octave, which you might have heard before.

Between two Notes of the same Name we divide the space into 12 Notes equally spaced called A A# B C C# D D# E F F# G G# cycling back to the A. We might also call them A Bb B C Db D Eb E F Gb G Ab, but that's a whole nother can of worms. We didn't always use the 12 tone equal temperament system, but that's how we do it now, because that way the most amount of notes appear, that sound good, "when played together with each other". It's just the most practical. The distance from one note to another is called an *Interval* with a specific name depending on how large the interval is. You might have heard about thirds, fifths, Seconds, Octaves etc.

Now, when you pick any Note as your Basis for any music (We call this our *root note*) we have to look at the intervals from the root to the other 11 possible Notes in our western System. If we just play them all, then it will sound really confused, so we gotta pick some of them. A group of picked notes to be played is called a "Scale". The A Minor Scale is A B C D E F G, the B Minor Scale is B C# D E F# G A, the A Major Scale is A B C# D E F# G#, the A Locrian Scale is A Bb C D Eb F G A and there are countless others.

After you got a "Scale" you want to base a song around you can say that this is the "Key your song is in" with the Root Note of the Scale beeing the "Key Center". If you now write a Melody in lets say A Minor you might go: A -> B -> D -> C -> E -> G -> A (Higher Octave) and have other Notes as chords to support the Melody. But you might Run into a Problem, because your Singer can sing the E, but not the G and high A requested, because it's too high. One Option would be to play every Note a whole Octave lower, with all the notes staying the same in name, but that just might sound way too low as a whole, so we have to change the Key.

For this example i said the E is the Note the Singer can still sing, so we move the highest Note in Question down to an E, which is 5 notes (Look at the full 12 Note-names here, not just the scale). A -> B -> D -> C -> E -> G -> A becomes E -> F# -> A -> G -> B -> D -> E (Higher Octave), which our Singer can likely sing. Congratulations, we have now changed the Melody to be in E Minor instead of A Minor. All the Instruments, Chords etc. have to also be played in the new Key.

*PS: Aside from Singers, Guitars (and other instruments) are also easier to play in certain Keys (A Minor, C Major, G Major, E Minor, E Major) than others (F# Major or Bb Minor). Piano is easiest on C Major / A Minor, because it's all the white Keys.*