Primary Triads
The primary triads in any key are tonic, dominant, and the subdominant. These fit into the T (tonic), D (dominant), and P (predominant) categories, respectively. Some theorists actually refer to the categories as T, D, and S, for subdominant.
In major, the qualities are I, IV, and V (all major). In minor the qualities are i, iv, and V. Sometimes, especially at ends of sections or works, tonic can be major.
Partwriting Primary Triads
Partwriting depends on the relationship between the triads – specifically the distance between their roots.
Chords that are a 4th/5th apart (fifth-related) will generally involve moving upper voices to the closest chord tones. Usually that involves keeping the common tone in the same voice.
Chords that are a 2nd apart will not have any common tones. You need to be careful not to write parallel P5/8s. The best thing is to move the upper voices in contrary motion to the bass.
Other Partwriting Issues
In minor, you always need to move down to the leading tone (#^7) to avoid augmented melodic intervals.
Always resolve your leading tone up by step. This is an absolute requirement if the leading tone is in the soprano. It is recommended elsewhere. If the leading tone doesn’t resolve up by step, it will move down to scale degree 5.
Never double tendency tones within a single chord. For now, this means that you will never double the leading tone in a V chord.
Harmonic Cadences
Tonic and Dominant represent opposite poles in the tonal system. Tonic represents your starting point, and your finishing point. Dominant represents the move away from tonic, and predicts its eventual return. Our names for harmonic cadences reflect this, and also take into consideration melodic cadence.
Harmonic motion that ends (a phrase) on the dominant (V) is called a half cadence (HC).
Harmonic motion that ends with the dominant moving to tonic (V – I) is an authentic cadence (AC). If the harmonic authentic cadence is accompanied by a conclusive melodic cadence in the top voice, then you have a perfect authentic cadence (PAC, or PA). If the melodic cadence is inconclusive, then you have an imperfect authentic cadence (IAC, or simply AC).
Harmonic motion that ends with the subdominant moving to tonic (IV – I) is a plagal cadence (PC). Plagal cadences occur much less often that authentic cadences. Some people refer to the plagal cadence as the “Amen” cadence, as it is sometimes found at the ends of hymns, after a perfect authentic cadence, where the choir sings A-men.
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