First, why is it important to be able to establish the key? If you have a room full of people and you want to sing a song together, you must all start on the same note or you will have chaos! If you have ever sung Happy Birthday with a crowd of people, you know how confusing an experience that can be with everyone starting in a different key!
So, how do you establish the key? The dominant 7th chord or V7 is ideal for establishing a key, which would be a G7 in the key of C major or minor, the notes G – B – D – F. But there is an even stronger way to establish the key and that is with a Perfect Authentic Cadence. This is a chord progression as follows: I – IV – I6/4 – V7 I. So, in C major that would be CM – FM – CM with a G on the bottom – G7 – CM. There is no doubt where the tonic or “Do” (the first note of the scale) of the key is when hearing this progression – even with Happy Birthday! Although, since Happy Birthday begins on the dominant (G in the key of C Major), an additional V7 (G7) at the end of the cadence will assure your crowd sings in key!