A dominant is a 5th above the tonic of the key and a subdominant is a 5th below the tonic of the key. In the case of C major for example the dominant would be G: C D E F G In the case of the subdominant we count below or backwards as in: C B A G F thereby giving us the subdominant of F. The subdominant chord is built on the F or the subdominant of the key. In the case of C major the dominant chord is G major and the subdominant is F major. The subdominant chord in the key is also known as the IV chord and is less tense than the V.