Music resembles language. It has its grammar and syntax. Any musical composition can be divided into phrases and sentences. A musical note is the simplest element of that language, an interval is a relation of notes, a scale is a set of melodic intervals and a chord is a set of harmonic intervals. Voice leading explains how to connect chords most harmoniously. The best way to study rules of voice leading is four-part writing.
Four-part harmony was widely used in liturgical chorales during the Baroque era. Since it was originally intended for vocal music four-part texture implies four voices. The highest voice called Soprano (sometimes it is also called Discant) approximately ranges from C4 to G5. The second upper voice is Alto: G3 - D5. The next one is Tenor: D3 - G4. And the lowest voice called Bass ranges from F2 to C4. In instrumental music, the division into voices plays a conceptual role.
Every tone of a musical chord represents an individual voice. So in the chord progression, there are several simultaneous melodies. It is called counterpoint. Types of contrapuntal motion are: similar - voices go in the same direction; parallel - voices move in the same direction keeping the same interval; contrary - voices go in the opposite direction; oblique - one voice stays in the same position and other moves.
Initially, musical chords are in the form of a triad. To get a four-part structure another chord tone needs to be added. The most logic option will be to double the tonic. However, doubling other chord tones can be acceptable too. Chords exist in two positions: open or close. In close position, upper voices lie thirds or fourths apart. In open position, upper voices lie fifths and sixths apart.
The connection of different chords can be: harmonic when the common tone of two chords stays in the same position or melodic when every tone does not stay in the same position. Doubling the tonic and not making steps more than a third we have next conditions of chords connection.
When roots of the chords lie fourth or fifth apart (e.g. I-IV, i-iv, I-V, i-V) their connection can be harmonic or melodic. Moving upper voices go in the same direction.
When roots lie second apart (e.g. IV-V, iv-V) connection can be only melodic because the chords have no common tones. Three upper voices go in the opposite direction to the bass.
When roots lie third apart (e.g. I-vi, i-VI) connection can be harmonic or melodic.
Always avoid parallel motion with next perfect intervals: unisons, octaves and fifth. The exception will be intervals with the same pitches. This motion is undesirable because it is considered too simple harmony.
Do not use augmented melodic intervals in any voices.
Start and end the composition with the tonic triad.
Do not use the same chord on the strong beat that was used on the previous weak beat.
In the bass, avoid two melodic intervals in a row that equals fourth or fifth.
Never double the leading tone. Doubling of it will cause parallel octaves because the leading tone always must be resolved to the tonic.
Usually, diminished triads are used in the first inversion.
The progression of several chords that lie third apart sounds weak.
It is desirable not to leave the bass on the same note moving from the weak beat to the strong one.
While augmented leaps are not useable descending diminished leaps are preferable than ascending ones.
Try to avoid parallel motion with major thirds.
Avoid leaving a lower voice on a higher pitch than a higher voice, crossing voices or overlap adjacent voices by more than one chord in a row.
Do not exceed an octave in three adjacent upper voices. Usually, the spacing between bass and tenor is not greater than two octaves. Try not to write outside the range of a particular voice. Because this way it is vocal-oriented. However, in freestyle especially in instrumental music, the ranges may vary.
Try to avoid similar movement to the perfect interval in outer voices (soprano and bass). Some theorists believe that it may sound like parallel perfect intervals.