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I have been searching for a while to try to correctly understand this.

What exactly does it mean if for example a song is written in C Major? Does it mean the song's notes cannot contain any sharps or flats? Does it mean the song can only have chords from ABCDEFG but use the major chords of those scales which could mean sharps and flats?

Any explanation on it would be really helpful.

Thanks

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If I'm reading music, it helps to know what key a song is in so I can play without glancing at the key signature on the left almost constantly. (I'm a very slow reader. I rarely read music. I only learned to read it because I played trombone in an orchestra when I was a teenager). To write music in a certain key, I can't answer, the key I write a song in is purely accidental (no pun intended!). –  Lee Kowalkowski Apr 10 at 21:11

7 Answers 7

up vote 29 down vote accepted

To understand the answer to this question you need and understanding of these concepts:

  • Key center
  • Tonality
  • Chord progressions in functional harmony
  • Cadence

A song is regarded as being in the key of C major if the pitch C is its key center, if the notes in the song chiefly fall in the C-major scale (as opposed to the C-minor scale, or one of the other scale-modes based on C), and the chord triad of C major serves the function of the tonic chord throughout the chord progressions in the song.

There are exceptions to everything. A song would generally be regarded as being in the key of C major if it started out in C major, even if it ended in another key. Some songs stay in the same key all the way through, and some songs change keys. Or it could start in C major but incorporate chords borrowed from other keys, use melody notes outside of the notes in the C major scale, or it could leave the key of C major entirely at one point, only to return to C major before it ends.

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So what is the advantage of even saying a song is in a particular key? –  bobobobo Jun 6 '12 at 1:20
    
bobobobo, I don't know about an "advantage", but the point of selecting a key for a piece of music is so that you can tell the band how you want them to pitch the song. –  Wheat Williams Jun 6 '12 at 3:07
    
@bobobobo See the last paragraph of my answer below for one such advantage. –  rishimaharaj Jun 8 '12 at 12:12
    
You don't actually describe what a "key center" is... –  naught101 Sep 4 '12 at 3:05
    
@bobobobo: The aswer by slim includes a list of 5 good reasons for choosing a particular key for a song. –  awe Sep 5 '12 at 6:28

I think it goes a bit stranger than a notion "of what chords you can use" and still be in a particular key, although of course that is an indicator.

Eg Brown Sugar by the Rolling Stones starts in G. Then the main riff begins on a Cmaj .. next chord is an Ebmaj which doens't fit CMaj at all, and the sung notes are generally in line with a Cminor, yet I would say the whole song is in C major because if you try playing a Cmin chord instead of C major, it just doens't work.

I think it's more about where the song "feels" like it wants to home back to (Cadence?), and making that something that you have to work out or wait for builds the tension that is the skill in good songwriting. Brown Sugar seems to 'want' to resolve to a C Major after the context of a verse / chorus.

Some songs have an ambiguous key, such as My Woman from Tokyo (Deep Purple) - verses are in Emaj, choruses in Gmaj. Which key is it in? I don't know :-) I guess at that point you have to just go with the first played chord which is Emaj.

That's for popular rock- classical music gets more complicated but I guess the notion of a feeling of a "home key" (ie chord) still works.

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There is a difference between "in C" and "in the key of C major".

A jazz tune in C can be ambiguous between several modes (like, say, parallel major and minor, or parallel mixolydian, dorian and natural minor, or all of these), with the common denominator being the root note of C. This root note sets the "tonality" of the music.

Similarly, blues in C does not necessarily mean major or minor. The tune could make parallel use of the minor and major pentatonic scales as well as the tritone.

The "key of C major" is somewhat more restrictive than just the tonality. It suggests that the music is probably organized around traditional concepts of harmony rooted in the European classical tradition. (An empty key signature, by the way, does not imply the key of C major; it could be A minor, of course. If harmonic or melodic minor occurs, the F# and G# appear as incidentals.)

The key or tonality might apply to just the opening bars of the music. Music which modulates to another tonal centre is still usually identified by the starting tonality. Often, composers return the music to the original tonality, but not always.

For example, in Baroque music, the common two-part form often goes from I to V in the first part, and the second part returns from V to I. The overall piece is identified as just being in I major or minor. The excursions into other keys are not acknowledged in the basic identification. So such and such a composer's "sonata in such and such minor key" can easily go through all twelve major and minor keys. It's possible for a multi-part work to be identified in C major as a whole, but to have entire movements that are not in C major. An example of this is Vivaldi's "Concerto per Flautino" in C major. (RV 443). The opening and closing movements are major (at least in their beginning and ending). The slow middle movement is in A minor.

Furthermore, a piece identified as C major could stay in C, but make small "parenthetical" modulations, such as the use of applied (a.k.a. "secondary") dominants. This is a harmonic device whereby the composer pretends that a target note or chord which is within the key is actually the tonic note of its own key. The composer then writes a cadence which resolves to that note. The cadence could just be a V-I (where the I is not the surrounding key, but the target note), or something somewhat more extended like a II-V-I.

For example, you can definitely work an A7 chord into C major music. It can create tension which resolves to Dm. The chord progression C A7 Dm G7 can be used as the basis of a C major song.

The A7 is just bit of "color" that is outside of the scale, but not a full-blown modulation. Identifying your music as being in the key of C major does not preclude the use of some sharps and flats which are not key changes.

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This answer is separate from my other one because it attempts give a more basic answer, to meet the bounty request.

What exactly does it mean if for example a song is written in C Major? Does it mean the song's notes cannot contain any sharps or flats?

In the most basic sense, yes, this is what it means. The melody and the chords will be constructed only from notes in C major scale -- and those are the white notes on a piano; no sharps or flats.

Does it mean the song can only have chords from ABCDEFG but use the major chords of those scales which could mean sharps and flats?

No. The chords will also only contain notes from the C major scale.

So the major and minor chords you have available are: C, Dm, Em, F, G, Am, Bm. These are the chords you get when you start on the note denoted in the chord name (C, D, ...), and play the third and fifth notes from there within the C major scale.

Now, if you see a piece that has no sharps or flats, that's not enough to tell you whether it's in C major or A minor -- since both of those are played with no sharps or flats. To decide which it is, you must work out what the tonal "centre" of the melody is. This is a matter of feel, so it's not possible give a strict definition. But it's often (but not always) the first note of the melody; even more often (but not always) the final note of the melody; and it's a note that will often end phrases within the melody.

So, if a tune has no sharps or flats, and seems to have a "home" at C, it's in C major. If it seems to have a "home" at A, it's A minor.

If its "home" seems to be a note other than A or C, then the tune is in a more obscure (in Western music) "mode"; I suggest avoiding that topic until you are comfortable with the basics.

The reason some of the other answers have been less clear, is that with more advanced pieces, things get more complex. If you sneak (for example) one Bb into a tune that's otherwise in C major, it's probably still C major. That's why we have to use words like "mostly".

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Interesting answer. So does that mean you can actually use any chords that are made up solely of C major scale notes? for example, I could play an A7sus4 (A,E,G,D,E) instead of A minor, and that would still be in the key of C major? –  naught101 Sep 4 '12 at 3:11
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Yes, any combination of the notes CDEFGAB is in the key of C. You can hold them all down together if you like (it doesn't mean it'll sound good). –  slim Sep 4 '12 at 10:23
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By the way - A7sus4, since it contains neither C nor C#, has no indication of whether it's major or minor. To in a piece in C major, A7sus4 will feel as if it has a "missing" C. In a piece in A major, it will feel has if it has a "missing" C#. –  slim Sep 4 '12 at 10:38
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Additional note about slim's comment on the A7sus4: This is because the 3rd is replaced with the 4th (which is the definition of sus4 chords), and since the difference between major and minor is defined within the 3rd, the distinction is removed. It is often used to add tension to a chord progression, which is resolved by landing on the non-sus4 version of the chord. –  awe Sep 5 '12 at 6:45
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Thanks for clarifying that slim and @awe. I'll have to try it out some time in C. Also, that explains why A7sus4 sounds so damn good in E minor. –  naught101 Sep 5 '12 at 7:46

Simply put, a song in the key of C Major will mostly contain notes from the C Major scale.

  • It can contain sharps and flats.

  • It can change keys in between.

  • There can be other anomalies.

Music is a hearing art. When we write down the music, we base it on a set of rules/traditions/formats. For example, the same note that we play on the piano can be called a G♯ or an A♭ on paper: it's just based on convention.

Having these conventions makes it easier for musicians to pick up the piece and play it. When they see 'C Major,' they will understand where the majority of the action will take place, and how the music should sound. Based on the notation, they'll not only be able to play any sharps and flats, but also will have a solid reference and will not have any confusion about what's going on in order to recreate the music themselves.

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First, you really need to have a good understanding of what major and minor scales are. Once you know that, you can more easily understand chords. Then, you should study basic chord progressions for each key, which are based on scales. You are not necessarily limited to the sharps and flats in a given key just because you are writing in that key, but you will generally stick to them, which means you are writing music in that key. Writing in C Major does mean there are no sharps and flats, but it is possible to have sharps and flats in the music by means of accidentals. It sounds as though you would really benefit from getting a private teacher out taking some theory classes. Good luck with your endeavors.

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Both the existing answers are good. I want to add that depending on your definition of "song", it may not even have a defined key.

If you sing Yankee Doodle and you start on a 'C', you're singing it in C major.

If you sing Yankee Doodle and you start on a 'F', you're singing it in F major.

The song doesn't have a key; your performance of it does.

(Note: not all tunes begin with the root note of the key they are in. If you play the melody from Morning from Grieg's Peer Gynt, and you start on a 'G', you are playing it in C major. If you play that tune and you start with a 'C' you are playing it in F major.)

When you perform a song, you necessarily pick a key, the moment you play the first note.

When you write down a song, in most notations, you necessarily pick a key, because the notation requires that you write down absolute notes. Exceptions to this include Nashville numbers and Relative Sol-Fa

Reasons for picking a particular key include:

  • the range of the singer or instruments
  • faithfulness to a particular performance/recording
  • ease of playing (e.g. D major or G major for beginner guitarists)
  • bringing out particular timbres on some instrument
  • bringing out particular subtle rhythmic inflections on some instrument

Often there are compromises to be made. For example, you are singing and accompanying yourself on guitar. You think your C major chord on the guitar sounds particularly nice. But that means you have to play a Bb major later in the song, and you always mess those up, so you play in D instead -- but in D you find your voice strains to reach the lower notes, so you try playing in E instead. The more voices and instruments are involved, the more complicated this gets.

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Just to clarify: Not all songs start on the base key note, although it is quite common (and it is so for "Yankee Doodle" in this example). It is even more common that a song ends on the base key note (again not always, but very common). –  awe Sep 3 '12 at 8:10
    
@awe worth noting, and indeed I'll edit to that effect. –  slim Sep 4 '12 at 11:44
    
I'll add to the reasons: particular notes/keys have their own feel separate from the timbre of an instrument. To me, at least, certain songs sound better in particular keys regardless of the instrument being used. And I particularly enjoy E (both minor and major) and I'm not sure why ;) –  Matthew Read Sep 11 '12 at 22:57

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