Chord Scales 

In any key, we use Roman numerals to notate the chord position and quality.  Major chords are in capitals, minor in lower case and the diminished chord is a minor-flat five (mb5) with a little zero. 

In the Key of D, the order of chords is as follows: 

D   Em   F#m  G    A7   Bm   C#mb5    D 
I    ii      iii       IV   V7   vi      viio          I 

The Sequence of Falling Fifths  

Chords usually progress in the circle of falling fifths, the same as the order of sharps:  F, C, G, D, A, E, B (say Father, Can, Go, Down, And, Eat, Breakfast). 

Starting on D, count down five:  D, C, B, A, G.  The sequence starts on I and goes to IV.  Continue counting out the circle of falling fifths: 

D  G     C#mb5  F#m   Bm  Em  A    D 
I    IV   viio         iii       vi     ii     V    I 

Most pop songs use the I, IV and V chords.  When you get into classical and jazz, you find sequences and chains of ii-V-I progressions (called "tonicization" or "creating tonal centers", to be covered later).