Seen these first handful of comments and ain't a lot of ELI5ing here yet, so here goes.

Here is a random number pattern example: There is no real difference between me saying 1-3-5-6 and me saying 2-4-6-7 (both start with a number, jump two, jump two, and then jump one)

The gaps between both sequences are the same. I'm counting the same sequence of gaps, regardless of whatever number I start with. That's what keys are.

Also, yes chords are groups of "numbers" we'll call them still, but now we are saying all at once instead of in a sequence. If "1-3-5" sounds good played at once, then "2-4-6" will too. The distances between the numbers are what's important, not the numbers themselves.

Imagine starting the count at 1 is A, starting at 2 is B etc etc.

You are right that songs are buttons pressed in a certain order. However in the vast vast majority of cases, a song will not use all possible buttons on the piano. It will instead use a specific selection of buttons. This selection is what defines the key of the song.

An analogy with English would be with usage of letters. If you write text with only words that start with F, or text with only words that don't contain E, etc., you can create many different "flavors" of text that will feel different. This is similar to what's happening when a song uses a specific selection of notes for its key.

This video covers a lot of the basics, you might find it helpful: https://youtu.be/28ipYr_aKb8

Phew where to start…

I like your post, but there are multiple questions and a lot to talk about, however I‘ll address only the main one: What’s a key?

When you hear one frequency, like 440 Hz, your brain will see this tone as the key. Add a different frequency (another white button on the piano) and go back to the 440 Hz, your brain will like it. What your brain also does is comparing ratios to whatever it hears. The simpler the ratio the more you‘ll like it. 2:1 or 3:2 are very nice.

I‘m not sure if the brain is prewired to anticipate tones with simple ratios or if it is a learned skill. Ultimately your brain likes it if you go back to the first tone or the simple ratios of it and everything in between is the skill of a songwriter to provide you with a nice journey.

Every simple ratio note derived from the initial frequency belongs to an extent to the key of this initial frequency. You will not like random frequencies mixed with the small ratio ones.

If you wait a few seconds after the initial note you can select a random frequency and this will then be ne next new key without bothering you. But play them together or in short succession and you won’t like it.

I‘ve jumped over a lot stuff here and it‘s hard to explain music theory without sound, so here is a video with a detailed and visual description of this topic: https://www.youtube.com/watch?v=mdEcLQ_RQPY&list=PL618khw0A-t5A7TyZmxAux_v_C3O7rO2l

OK so, the keys on a piano are black or white, right? The white ones are A B C D E F G. The black ones are either sharp or flat, let's not worry about why for a second and just say the black keys are Ab Bb Db Eb and Gb.

If a song is in the key of C, then, very simplistically, almost all the notes and chord are *only* comprised of the white keys. That is, a "key" is 7 out of the available 11 notes. Those notes sort of "sound good" together and most western music is oriented around a key.

Different keys are different note, but the 7 notes in that key have the same relationship as in the key of C, just shifted up or down.

So, the "Do Re Mi" is a great launch for an example of a scale and key. As you step through it singing, each sound is a slightly and particularly higher note than the last right? If you do it again, but sing the first Do at a different starting note (lower or higher, doesn't matter) you'll step through the rest of that scale on the same pattern of rising notes. So here, you're singing the scale in a different key. Same sounds, same pattern, but different "base" note.

Songs are all in a key, that is rooted in a base note and a particular pattern. Another artist might play the same song in a different key for a stylistic reason or simply a preference. Like the cover versions of Landslide or Jolene.

An easy place to hear a key change is in a lot of older country songs, like Good Hearted Woman. That song is in one key for most of it and then transposes up a bit to a new key. They play the same pattern, but base it off a different key.

>Are songs not just "buttons" pressed in a certain order with certain delays (and pedals and whatnot)?

The rest goes into some more delicate theory points but this is something that's easy to address. Music is not just a series of instructions that players repeat like machines. Sheet music leaves out a lot of details because it would make it unreadable to include everything and because it is expected that players will add their own interpretation. How loud or soft something is played is a good example, a composer may specifically put in some places where they feel the dynamics need to be a certain way but players can add their style elsewhere or even ignore the composer completely.

If you want to go down a rabbit hole look up The Firebird Suite Finale on youtube. There are videos of the composer, Stravinsky conducting it, along with many of others throughout the years. You could argue that Stravinsky's version is the correct version but even between different videos of him you can hear changes and I would argue they aren't even the best sounding versions.

And then of course once you get into jazz and more out there genres players start improvising. In jazz you'll often get a sheet that has a quick melody to play at the beginning and the end, with the majority of the performance expected to be improvised based on the chords given. And even those chords will often be modified by musicians because they can.

I'm going to take a different approach, starting with chords, and moving to key.

You're right, chords are multiple notes heard at the same time. The study of how those notes interact is called Harmony. Chords can be simple, or really complex. But they generally we can separate them into two types: notes that sound good together are called consonant, and notes that sound like they are fighting or are unpleasant together are called dissonant. That's right, the word harmony doesn't mean pleasant, harmonies can be pleasant or unpleasant (consonant or dissonant).

Now, in a piece of music not all chords are equal. In the vast majority of the music you are exposed to is called "Tonal" music. This means that there is a note or chord which feels like home. Music is like writing a story; if there isn't a final ending to the story, it just feels wrong. As you said, you are aware of do re mi fa sol la ti do. That's a great example. If you were to sing or play those notes, and stop on ti, it can be painfully incomplete. I like to demonstrate this in class. I'll play those notes and stop before the final Do, and the whole class squirms, or tells me "Finish it!" I've even had students run up to the piano and play it themself.

You can think it like grammar and punctuation. Sentences can end in a question mark, or a period. One feels incomplete, the other feels complete. In tonal music, there are all sorts of ways to point to the chord or note which is "home." It's all about tension and resolution. It's a lot like writing a book. You introduce your main character (establish the key), go on some adventures (different chords, chords with tension), and finally conclude the story (returning to the home key).

Take note: this is a description of tonal, western music. India for instance, has a long history of its own classical music, that uses different expectations and tools.

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I spent six months of private study in my late 30s learning this. My husband assures me that most people have an intuitive understanding of what it means.

Thanks to all those responding to this question.

>How can songs be in a certain key?

In western music we have 12 notes in total: Do, Do# (Re♭), Re, Re#(Mi♭), Mi, Fa, Fa#(Sol♭), Sol, Sol#(La♭), La, La#(Si♭), Si

A key is basically a predefined set of these notes that we have to play on so we can stay in that key. The more music theory you know the better you'll be able to break this rule and even change keys at the middle of a song.

>How are there like keys and majors and like C minor chords and whatnot?

Keys are not just made up randomly, they follow certain patterns such as how many tones or semitones are between each note. This pattern will decide if it is a major or minor key.

We already established that a key is just a set of notes from the full spectrum of notes available. A chord is basically the same thing on a smaller scale, they are a set of notes you take from the key you are playing on. Same as in a key, how many tones and semitones divide the notes from your chord will also determine if it is major or minor chord.

>Does that just mean the voice sounds like a music note?

Yes. For example, a human voice and a piano may both emit the same note (same frequency), but you can distinguish them because they both have different timbres.

>Are songs not just "buttons" pressed in a certain order with certain delays (and pedals and whatnot)?

If we are oversimplifying it, yes. However, if we want our song to sound good then we need to follow certain rules. If you know enough theory it is possible to create a nice simple song on paper before actually hearing it.

Every answer can be elaborated way more than this but I tried to keep it in a ELI5 as much as possible.

There are a lot more notes than c d e f g a b, they all have sharps (bit higher) and flats (bit lower)

When a song is in a key, we mean that it has a specific note as its base, and the other notes follow that notes scale.

A scale is a sequence of note distances, so no matter what note you play, the next note in a scale will always be the same distance. Think of it as a staircase that has a sound for each of its steps and when climbed up it always follows the same sound pattern.

A C major scale is played with all of the white tiles of the piano, from C to the next C. A song in C major will use those notes. Another scale, like G major, will need to use one black tile in order to keep the same pattern.

Basically, a scale is a pattern of sounds, and the key is just which note is being used as the first note in the pattern.

I'm late to the game but bringing my band geek knowledge to try and ELI5.

The white and black things you press on a piano are called keys, but that's not what it means when we talk about the KEY of a song. If you made a song using all of the strings on a guitar or all of the keys on a keyboard together randomly, it would not sound like MUSIC, it would just be noise.

If you make up a song and sing it to yourself, you could then go over to a piano and pick out what the notes are, and write them down. There will be a KEY that is most natural for that collection of notes. For instance, if the notes you wrote include a lot of B flats and E flats, your song's natural KEY is the B Flat Major Scale, which goes B flat, C, D, E flat, F, G, A, B flat. (When you are learning any instrument including voice, you will learn all the major and minor scales.)

So a song is just a series of notes with a set relationship between them. You could transpose the song you just wrote (change the notes, keep the relationship between notes the same) into any KEY, although it might change the mood of the song. Changing the key is useful for singers if the song in its original key has some high or low notes they can't hit. But if you want to play music with more than one person/instrument, you all need to be playing in the same KEY.

One thing I will add is that even songs that use the same notes can sound very different based on where the song starts.

C Major and A Minor use the same notes, yet sound very different. To illustrate:

1. My Girl - The Temptations, is in C Major.
2. Midna's Lament - Twilight Princess, is in A Minor

My Girl sounds happy, while Midna's Lament sounds melancholy.

Keys are the name we call certain groups of the ‘buttons’. If a song uses only or mostly buttons from one of the key groups, then we say it is in that key.