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I am just starting out to learn some music theory since I want to play by ear and I noticed some terminology and it's confusing me.

When someone says a song is in the C major key (which are notes C,D,E,F,G,A,C) is that the same thing as the C Major Scale? What is the difference between C Major key and C Major scale?

Also does that mean that in this specific song, if it is in the C Major scale/key that I can only play chords in my left hand which are derived from the C major scale/key? So I can't play chords from the D major scale or the A# major scale with my left hand?

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    "So I can't play chords from the D major scale or the A# major scale with my left hand?". Chords don't necessarily need to be "from" a scale: they exist in their own right.
    – Some_Guy
    Oct 9, 2017 at 12:13
  • Don't expect theory to tell you what notes and chords you MAY play. Read and play lots of existing music. See what good composers and musicians HAVE written and played. Use theory to collect and categorize a 'bag of tricks' for use in YOUR music.
    – Laurence
    Jan 7, 2018 at 12:37

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does that mean that in this specific song, if it is in the C Major scale/key that I can only play chords in my left hand which are derived from the C major scale/key? So I can't play chords from the D major scale or the A# major scale with my left hand?

You can play any chords, even those outside the key. The rule is: If it sounds good, it's good. You'll notice that songs written in a specific key tend to use mostly chords from that key, but other chords are regularly peppered in there too.

When someone says a song is in the C major key (which are notes C,D,E,F,G,A,C) is that the same thing as the C Major Scale? What is the difference between C Major key and C Major scale?

C Major Key is indeed related to C Major Scale. As you continue studying Music Theory, how they relate will become very clear.

Music theory is worth studying. Remember: It is not a set of rules telling what you can and cannot do. It is instead a way of looking at music to understand what is happening and to communicate with other musicians about it.

I studied it through a youtube playlist by Ben Levin called "Music Theory from the Ground Up". I recommend it if you don't have other material.

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  • Yeah I looked through his playlist I pretty much know everything from that playlist tbh. I just want to now know an effective way to select what scale to use for a specific song and which scales work together. Really broad question haha but I guess I will just keep practicing... Very annoying initially :)
    – CapturedTree
    Oct 9, 2017 at 6:25
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    If you mean you are creating a new song and wonder how to pick a key for it, then any key is fine, as long as it sounds good to you, and as long as you can play the chords in it. If you mean you are playing an existing song by ear, then find the "tonal center", and that note will be your key. Example of how to find the tonal center: youtube.com/watch?v=a4sh6OuOMZo
    – Max D
    Oct 9, 2017 at 6:49
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The key of C is clearly related to the scale of C. That's where 'C' comes in. Called 'C' because that is the hub, centre, home, et al of both the key and the scale. In Western music, there tends to be a centre to a lot of music - it's somewhat like a journey. It starts at home, goes on various visits, and eventually returns. This can often (but not always!) be felt in the music itself. The beginning is obvious, and establishes 'home', and at various points along the way, it will return to that 'home', and could even be construed to finish there, at that very point. Of course, it's often a temporary drop back into the house for something, and we're off again.

The main notes that are used for this most often come from the set called diatonic, and all belong naturally to that specified key, so when put into order, as humans like to do, they constitute a scale.

Just as when on a journey, we sometimes take a detour, music can go off at a tangent, usually just a little, and move temporarily into another key, often related to the original, thus changing only a few notes. Modulation is its name, and just like the journey, it gets back home via a different route.

The majority of pieces will be in 'a key', and thus use the diatonic notes from that related scale to sound that way, but sometimes a twist in the journey will necessitate other notes (which by definition are from other keys/scales) being used. So just because something is 'in C', doesn't mean it's restricted to using only those seven notes that constitute that key or scale.

Chords? They're closely related to the scale notes, and a lot of the harmonies in a piece will reflect that in the notes making up those chords. But other notes can and do get used. Out of the 12 available, 7 are diatonic, leaving another 5 to colour the tune and its harmonies. Thus chromatic. Serendipity on the journey! And, as oft said - if it sounds good, it usually is! Theory can explain that, but does it really make any difference?!

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  • Depends on the type of music you're talking about but using only notes and chords built from the major scale of the key would be the exception not the rule. I think the idea that the major scale is the "basic unit" of music and other notes are "borrowings" is harmful to the understanding of, say, rock music. In this sense, a small amount of "theory" is probably more harmful to compositional ability than none at all; trying to write rock music based on a basic understanding of traditional diatonic harmony is like trying to write Mozart with Guidonian hexachords. It just missed the point.
    – Some_Guy
    Oct 9, 2017 at 12:26
  • @Some_Guy - you're right, of course. I addressed the OP generally, and rock is but a small portion of music, (< 2 or 3%?), probably not coming under the term 'general'. I've played rock, etc. for many decades, and it doesn't follow general theory lines, especially regarding scale structure! Rock wouldn't work at all if it followed diatonic guidelines!
    – Tim
    Oct 9, 2017 at 13:53
  • I picked rock music as an example not because it's a notable special case, but because 1) it's familiar to most of us 2) it's isn't usually thought of as high-brow/deviant/complex music 3) I'm currently writing an answer to a question of yours about chord function in rock music, so it's on my mind. My issue is with a sentence like ; "The majority of pieces will be in 'a key', and thus use the diatonic notes from that related scale to sound that way, but sometimes a twist in the journey will necessitate other notes (which by definition are from other keys/scales)", for 2 reasons (cont.)
    – Some_Guy
    Oct 9, 2017 at 19:48
  • 1) that only using notes from the diatonic scale reflects the majority of pieces (however you want to characterise it geographically or time-wise: it's not even true for say the majority of western music from the last 100 years) 2) that any use of notes or chords outside of the major scale necessarily represents "borrowing" from another key or scale. I'm not saying learning about basic diatonic harmony isn't valuable: after all, you have to start somewhere. But the idea that this represents the "normal" rules which are "occasionally broken" is worse than useless to understand most music.
    – Some_Guy
    Oct 9, 2017 at 20:01
  • @Some_Guy - Would it not be better if you provided a separate answer here, as it seems I have so much wrong? It's a great opportunity to put it all right, and another aspect on the question will never go amiss.
    – Tim
    Oct 10, 2017 at 7:20

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