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
To understand the answer to this question you need and understanding of these concepts:
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.
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:
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.
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.
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".
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.
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.
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.
I was taught in college music theory class that a key is defined by the tonic/dominant7 chord relationship. For example, C G7 in C major, or c(minor) G7 in c minor. A piece in C major might not use G7 chord, but G chord, it would still be in the key of C major, but the G7 chord could be used and still be in the key.
If a g minor chord was used in C this would indicate C mixolydian mode. If a g minor was used in c, key signature 3 flats, this would indicate that the piece was modal i.e. c aeolian. A g minor chord is within the key signature, but the 'key' of c minor uses a G7 or G major chord for the dominant chord.
If a song is in a key of C major, it cannot contain any sharps or flats, not even in the chord. As it follows in the major scales,WWHWWWH and the chords, I ii iii IV V vi vii° (The capital letters are major, the small ones minor, and the one with the circle thing is diminished.) It means C major, D minor, E minor, F major, G major, A minor is how you play C major key. C major and it's relatives are the easy keys to play in because you just play all the white notes.
Now, to answer your question, to write a song in a certain key (all songs have their keys) means the notes that are going to be used are the only ones the key is in. So for example, Am, a relative of C major to make it easy, all white notes are the only ones that are going to be used, however there are exemptions. If a song has key change, for example A minor to B minor, well then as soon as the key has changed then the notes has changed, too. The minors formula is WHWWHWW and if you look at that carefully at your instrument it means that B minor has C and F for its sharps. Then you have the notes the song is going to use. I recommend you study scales so music will be a lot easier. I hope this helps and I hope my post isn't too complicated. =)
