Ive been playing the piano for a bit and I am now looking to learn new scales. I've run into a question and I'm quite stumped. For example, I've seen the D major scale as D – E – F# – G – A – B – C# – D but I've also seen it with the C#dim instead of C#. So I would like to know how I am supposed to play it. I've also seen that the diminished note seems to pop up in every other scale as well. I play the notes as triads so I'm really confused.
In tonal harmony, all major (Ionian) scales follow the same pattern of whole steps and half steps: W-W-H-W-W-W-H.
When you want to play all of the major triads for a particular key, you stack each chord up in thirds and use the notes from that specific scale.
For example, if you are playing the D Major scale, the D triad would have the notes D-F#-A; D to F# is a third, and F# to A is a third, and those are the notes in the key signature. You wouldn't play D-F-A because F-natural isn't part of the key of D Major.
I don't know how well you know intervals, but basically it is the distance between two notes. D to F# is a third (you count the note you start on as 1, so D to E is a second, D to F# is a third, D to G is a fourth, etc).
More specifically, D to F# is a major third (a major third is 2 whole steps/4 half steps apart). F# to A is a minor third (a minor third is 3 half steps apart). The resulting triad would be a D Major triad.
The reason this triad will be a major triad is because it is stacked up with a major third on the bottom and a minor third on the top. All major triads are built this way, and if you play what is known as the I chord or tonic chord in any major key, it will be a major triad.
Take the next triad in this scale, whose root is E. It would be built this way: E-G-B. E to G is a minor third, and G to B is a major third. This results in an E minor triad. The ii chord in any major key is a minor triad.
If you do this for all of the triads in the key of D Major, this is what you would get: D Major (I), E minor (ii), F# minor (iii), G Major (IV), A Major (V), B minor (vi), and C# diminished (vii°).
When you are building a chord in the key of D major starting on C#, you would get the notes C#-E-G. C# to E is a minor third, and E to G is a minor third. Diminished triads are the result of stacking two minor thirds on top of each other. In any major key, the triad built on the seventh scale degree will be a diminished triad.
As a side note, this "leading-tone triad" has a desire to resolve to the tonic (I) chord; this is because it has the leading tone (the seventh scale degree) as its root, and it has a tritone (an unstable interval 3 whole steps apart) between the root and the 5th of the chord that really wants to be resolved. If you play this on the piano, you will be able to hear this for yourself. In the case of the key of D Major, the leading-tone triad is the C#-E-G chord; the tritone (C# and G) wants to resolve to the notes D and F# from the I chord.
This is the pattern for all major keys represented two ways:
I ii iii IV V vi vii°
M m m M M m d
And as a final reference, these are the 4 types of triads and how they are built from lowest to highest:
Major Triad: Major third, minor third.
Minor Triad: minor third, Major third.
Augmented Triad: Major third, Major third
Diminished Triad: minor third, minor third.
The augmented triad is the only one that doesn't naturally occur in major or minor scales. This chord is used more sparingly in music, and it has its own functions and purposes for when it is used, but that goes into a little more advanced theory.
I hope this makes sense and helps!
Unfortunately, notes and chords (triads especially) are often called by the same names. When someone says 'play a D', it could mean play a D note, or play a D major chord.That's confusing, and it's where you came unstuck.
You already know the note names for the D major scale, that's fine. The first answer explained the triads. From a major scale there's always three major (I, IV and V), and three minor (ii, iii and vi) triads all built on the notes from that scale (no matter what key). They're called diatonic chords. That just leaves one more triad - viio. Unlike the others, and because of where the notes are in the scale, it has two m3 intervals, and is, as you say, called diminished. It's probably the least used of all seven, although it can be seen as an extension to the V triad.
In key D, the V triad is A, C♯, E. The viio triad is C♯, E, G. Overlap them, and it gives V7 - the dominant chord A7 in key D.
You mention 'diminished note'. There is no such thing! C♯ is the leading note in key D, and the triad using that as the root note is called a diminished chord. There are also diminished intervals, but they don't really need discussing here, save to say C#o actually has a diminished 5th in it (C♯>G) - hence its name.