It would be appropriate, at this time, to begin relating the previous and future melody examples with Key Centers. We have covered three Key Centers in the previous melody examples. In common terms we use phrases like "Key of C", or "Key of G" to denote Key Centers or Tonal Centers. All are valid statements.
Three key centers are listed above. In the diagram, common chords associated with these centers are defined. In addition, two naming conventions are provided for each chord group. The "Root" or primary chord of each center is named the Tonic or I chord. Next the Sub-Dominant or IV chord is shown. Finally the V7 or Dominant 7th chord is shown. These names are communicating a specific chord within a specific Key Center.