In music, a key is a family of tones whose regular members are called diatonic tones, and named key tone (or tonic) or one (or eight), mediant or three, dominant or five, subdominant or four, submediant or six, supertonic or two, and subtonic or seven. Chromatic tones are temporary members of a
key, under such names as sharp four, flat seven, etc. Scales and tunes of every variety are made from the tones of a key. The name key is also given to the fundamental tone of a movement to which its modulations are referred, and with which it generally begins and ends. Its keynote.
In music the mediant is the third above the keynote. It is so called because it divides the interval between the tonic and dominant into two thirds. Research Mediant