If you go through all the scale tone chords in the key of C major you come up with the following pattern:
This pattern remains the same regardless of the key. For any major key, Chord I is always major, chord II is always minor, chord III is always minor, etc. The only thing that changes from one key to the next is the letter names of the chords. This can be demonstrated by looking at the scale tone triads for the key of G major which are shown below.
As mentioned earlier in the book, transposing means changing the key of a piece of music. By simply following the roman numerals and remembering which chords are major, minor, etc, it is easy to transpose chords from one key to another Here is the progression from the previous page transposed to the key of G.