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From what I read, the key of a song or piece of music can be either from the major scale or natural minor scale.
Is this true? Can a song have a key in a different scale?
I ask because there are also "modes" - can a piece of music be in a certain mode as its "key".
I am confused as to what scales can be the "key" of a song and more importantly why it works that way.
There is a lot of question wrapped into one here.
The key signature is indicated at the beginning of a piece on the sheet music. For example a song written in they key of C will have no accidentals, one written in A will have (F#, C#, G#) all indicated at the beginning.
This does not mean that the song must stay in that key throughout. It is very common for music to modulate key. A common pattern is to play a melody in the major scale of the key of a song then move it up to the 4th, Fa, of the same key. Some composers will move the melody without changing key, but will change mode, while others may introduce accidentals in the melody to indicate a true key change from the original, starting on Do, to the key a 4th up, starting on Fa of the original key. Not all scores show the key change. Some will opt to keep the key signature of the song the same and write in accidentals while others will opt to show a new key signature in the sheet music. In fact I have seen pieces written in the key of F or G major but the score shows C as the key. Thus the Bb or F# is written throughout. This is not just in beginner exercises but in professional scores. If the modulation is not going to last long I personally don't need to see the key change, it doesn't really help. In musical scores (scores for Broadway musicals), however, they key may change every few bars so it gets messy to keep digesting new accidentals. In this case an overall key change is more helpful. It's a matter of how the info is presented. You still have to digest new accidentals but if they come one at a time your brain will not comprehend an overall shift in key but when you see the key signature you "know" from years of training what group of accidentals come grouped together and for some instruments the change is just a matter of moving your hand to a new position.
My example of musical score serves to point out that a song can change key as often as the composer likes.
Modes are interesting. There is a relationship between the modes that you may be familiar with. The seven modes are all related to the major scale but starting on different degrees.
On degree 1 of the Major scale --- The Ionian mode (or major scale)
On degree 2 of the Major scale --- The Dorian mode
On degree 3 of the Major scale --- The Phrygian mode
On degree 4 of the Major scale --- The Lydian mode
On degree 5 of the Major scale --- The Mixolydian mode
On degree 6 of the Major scale --- The Aeolian mode (or natural minor scale)
On degree 7 of the Major scale --- The Locrian mode
For example, if you write a melody in D dorian you are really in the key of C major and the sheet music would be written that way. What makes it recognizable as a dorian melody rather than a Major melody is the starting and ending notes of the phrases. A classic example is So What by Miles Davis. This is written in D Dorian.
Now on to the idea of playing in a mode. In classical western music theory and tradition (as it has evolved to in this day and age) it is not wrong to say that songs are predominantly written using the major scale or minor (typically Melodic or Harmonic minor rather than natural minor). But when Jazz players call out So What at a jam session they will often say play it in D. They mean as written, in D Dorian. The first time that happened to me I transcribed up to E Dorian thinking that was the correct "Key", i.e. D Major. So, sometimes people do not refer to a song by the key signature but rather by the note on which the song starts or of the mode being used. You can analyze a modal piece from the classical western music theory perspective and that will work but some cultures are not derived from this tradition.
This brings me to another example where "Key" loses meaning, the Blues. Blues breaks the mold in more ways that one. We typically "change key" with every chord in the I--IV--V sequence by using doninant7th chords on each degree of the progression. In theory this would indicate using the corresponding Mixolydian mode at each chord position and some players do this. But many opt for using the MINOR pentatonic scale starting on the I note over all 3 chords and that works too. You are "out of key" strictly speaking but it's an OUT that works. The more useful scale is "the blues" scale which is minor pentatonic with an added b5. Even more useful is the same with a major 3rd added, a very common staple of Pat Martino solos (and other Jazz greats). Here you have a long chromatic run from the b3 up to the 5th. When blues players call out a tune they might say "The Blues in Bb". This usually (but not always) starts on Bb7 which is, strictly speaking, in the key of Eb major. Written out in sheet music it may very well show the key of Eb but that's not how we think of the blues. The blues in Bb is probably written in a Bb minor mode or scale and played over an Eb major chord structure that modulates key with every, or almost every, change.
One could apply western classical music theory to the blues and map out all those changes and indicate them in a score with appropriate key signatures and whatnot. But in the end that would probably be more confusing for a player who knows the blues tradition.
Not all music is western classical and even tough the rules of music theory and harmony theory are useful there are exceptions to those rules.
Key and scale are two different concepts, and physically, two different things.
I understand that beginners (especially!) get confused. A piece in key C uses mainly the notes from the scale of C, and in early stages, the two could easily be one!
However - the vast majority of pieces have a 'home' point, where they feel at rest. Regard it as often the start point for the journey. That will be the root of the piece. If it's, for example, A, then the notes A, B, C♯, D, E, F♯, G♯ will be, to Western ears, the most natural sounding notes that fit for that initial journey.
To make life easier for readers, the composer will write the 3 sharps at the beginning, to indicate that they are the 'notes changed from the norm of no sharps or flats'. The key signature. Most players at that point put on their 'A hat' and play in the key of A major. Yes, using mainy those notes found in the scale of A. Most but not exclusively. I mean, it could be in key F♯ minor!
Just like when we go on a journey, the music may take a detour. That could take into account that all those notes we thought (belonging to scale A) fitted, don't fit as well any more. The piece has modulated, and to incorporate that, a note or two needs to be changed. Chances are, we'll still return home at the end - it's where most journeys end - so the piece will (often) get back to the point where A feels like a good end point. But - it's used other notes apart from the scale notes (diatonics), on the way.
It's all a bit of a compromise, and, I guess, having some rules in place helps to keep players on the straight and narrow, those rules can be self-defeating if taken as gospel. They're not - they're purely guidelines.
You ask about major or natural minor for the key. Not so. A piece can be in any of the 7 modes, and can be in the relative minor, which have some other changed notes to the key signature, which complicates the issue more.So, the key sig. is a shorthand way to tell a reader a fact, but often falls short on information - with minors often not telling the whole fact.
AFTER ALL THAT: reading between the lines of your header - possibly if it read 'Can the key signature of a song be only major or natural minor?' - then even there, the answer is no, as that key sig. could designate a set of notes which could be used modally. Key sig. 3♯ may signify A maj., F♯m, B Dorian, D Lydian, for example.
From what I read, the key of a song or piece of music can be either from the major scale or natural minor scale.
People are often taught something like that, but as you have noticed, it is not always true.
'Keys' are one way of describing the tonality of a piece of music. As you say, the key system does assume that a piece of music will be based around the major scale or natural minor scale.
Saying that a piece of music is in a certain key does not imply that it only uses notes from those scales, so the idea of 'key' is more flexible than you might think. Minor keys in particular often use notes from other minor scales.
However, it is still true that the tonality of many pieces of music cannot be described using only the idea of 'key'. A very obvious example is blues music: notes in blues are typically bent and inflected within certain ranges, including the third, which is often bent between major and minor. So it's often impossible to say whether a blues piece is 'major' or 'minor' - it's neither, really.
And of course non-Western pieces of music might use completely different scales, and also not be easy to describe using the idea of key.
Basically, 'key' is an idea that works well to describe some pieces of music, but not others.
A song can be all and everything. Major and minor are just 2 special cases of modes.
The key defines which of the 12 tones is the root tone and from there we can derive the mode and tonality gender of a song.
...music can be either from the major scale or natural minor scale...
Whatever you read is mixing up key signature with key and scales.
The statement is sort of a quick and dirty way to describe the basis of keys and harmony, but it can easily lead to misunderstandings, especially with minor key and chromatic harmony.
The best thing to do is stop thinking of keys and key signatures as scales, because they are not scales.
If you have a piece of music in G minor, it will have a (modern) key signature of two flats (B flat and E flat.) The various tones that will be used in the music - in common practice G minor - will include the different tones of the natural and melodic minor scales. To find a very concise description of how minor key harmony works look up the rule of the octave and check the minor key examples. In the space of a few bars of music you will see the harmonic conventions of minor.
Modal music is more complicated, because there are several different styles of music that use the different modes in different ways: Renaissance music, classical, jazz, folk, rock, etc. Probably the most practical thing for you to do, from a pop music perspective, is look up chord borrowing and look at the chords in a book of folk music. Certain patterns like i bVII i or I iv I etc. are common ways that modes come up in music.
It seems like you should get a good college harmony textbook and with that in hand start analyzing examples of real music.
Try to think of a key centre as a foundation for which to build complexity upon. If most of the notes are within a particular key (diatonic) then the music is usually considered to be in that key. From that foundation one can, by raising or lowering specific notes, move to other modes or the relative minor of the determined key. It's important to consider the first and last chords/notes of a piece in determining what key it's in. If they differ you are probably witnessing a key change. Of course, the music can begin and end with a relative minor chord or note of a particular key and be considered minor (a raised 5th note of the relative major key would certainly confirm this). For most music this is an obvious process, but for some music there is no clear consensus on what key it is in. It's always important to remember that 'music theory' is like a science. It's a way of explaining a a natural phenomenon.
Strictly speaking no, the scale and the key are two different things. E.g. if a song bases on an F-dorian scale, and its tonal center is F, then the key is F-minor.
However, I don't believe it's incorrect to say "a song is in F-dorian", when you want to emphasize what scale is used, and it implies the key is F-minor.
