Question: How do I know which notes of a scale may be a Major/Minor chord?

Take [this](https://music.stackexchange.com/questions/6177/finding-out-which-scales-contain-a-specific-chord) question as an example. The OP states that the C major scale contains the following chords: C, Dm, Em, F, G, Am, Bdim

How do I accomplish this?

Answer: 
What OP means is that if you use the notes of that scale (no sharps or flats added or removed) and play a third and a fifth on each note you get those chords.

**Example on C major scale:**

C (first note):  

3rd up = E  

5th up = G  

Chord = C major 

D (second note):  

3rd up = F  

5th up = A  

Chord = D minor (note the half step between E/F making the `minor third`)

G (fifth note):  

3rd up = B  

5th up = D  

Chord = G major

A (sixth note):  

3rd up = C  

5th up = E  

Chord = A minor (note the half step between B/C making the `minor third`)

B (seventh note):  

3rd up = D  

5th up = F  

Chord = B dim (note the half step between B/C making the `minor third` and also half step between E/F making the fifth not perfect like all others but `dim`)
This may have already been answered, but I'll throw in my grain of salt. If you remember, every triad is made up of 3 tones. The root, the third, and the fifth. If you play a C on your piano, then play one third up, and then play a third above that, you have a C major chord, because C-E is an M3. 

Continue and you get an m3 in D-F, 

an m3 in E-G, 

an M3 in F-A, 

an M3 in G-B,

an m3 in A-C

and an m3 in B-D. 

The reason that vii is diminished is because b-d-f is TWO m3s stacked on each other. This works in **EVERY** major scale. M is Major, and m is minor.