It is common practice to describe all the chords within a key with roman numerals. If you go through and analyse all of 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. This means that if you look at the scale tone triads in any major key, Chord 1 is always major, chord 2 is always minor, chord 3 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.
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 scale tone triad exercise from the previous page transposed to the key of G major. Once you can play it in this key, play it in all keys; both around the key cycle and chromatically ascending and descending across the range of your instrument. Make a habit of doing this with every new thing you learn.