I have alluded to this in a few other posts, but I've never gone into much detail, or provide examples. Hence this thread, an introduction to melodic construction and the principles of voice leading/part writing. The principles typically refer to (18th century) 4-part harmony, but are applicable to many different styles.
In music, individual lines or parts are often referred to as 'voices'. (In 4-part harmony, the 'voices' are generally called Soprano, Alto, Tenor, and Bass, in that order, from highest (top) part to lowest (bottom) part.) Hence part writing and voice leading generally mean the same thing. 'Part-writing' is the original English term, and 'Voice-leading' was adopted in America from the translation of the German word Stimmführung. Now, the terms are generally used interchangeably in both cultures.
Voice leading/part writing is the relationship between consecutive notes in voices/parts. It is an established laying-out of notes such that each individual part is as smooth as possible and sounds pleasing to the ear. A good melody will certainly employ these principles, but voice leading is not confined exclusively to melodies, it refers to all parts, including (and perhaps, especially) the bass.
When writing (horizontal) parts:
In all parts, there should always be more stepwise motion than leaps. The best melodies contain a mixture of steps and small leaps.
The next note in the part should be as close as possible to the note before it. - In other words, when writing parts, move to the nearest note possible.
If two adjacent chords have a note in common, keep that note in the same part.
Do not cross parts (for example, don't allow the tenor to rise above the alto, or fall below the bass).
Do not overlap parts. - A second note in a lower part should not rise above the first note in the higher part and vice versa.
Try to keep the tenor high.
There should not be more than one octave gap between any adjacent parts (soprano/alto, and alto/tenor), except tenor and bass, where a big gap is allowed.
Here is a piece of music containing examples of all the above points:
Note, the first bar is bad. - It sounds too random and plonky. The second bar is much better, - it sounds nice and smooth.
Leaps
Leaps can make a part more interesting, but should not be overused.
Never leap a 7th, diminished 4th, or any interval larger than one octave. Also, never have such an interval with only one note in-between. (If you are unsure about intervals, consult my Introduction to Music Theory).
Never leap any augmented interval.
Leaps of diminished 5ths, octaves and 6ths must be followed by notes contained within (in-between) the interval.
Note; these 'forbidden intervals' generally result from the leading note of the scale.
Other stuff
In minor keys, generally use the Harmonic Minor to harmonise with, but use the Melodic Minor for writing melodies (Soprano line). - Sometimes however, raising or lowering the 6ths and/or 7ths can get you out of a tricky situation.
Don't sandwich chords. By this, I mean don't repeat the same exact chord until at least two separate chords have come in-between (inversions though are fine).
The bass is the next most important part after the melody, so it needs care. Bass lines need to be smooth, almost like a melody in their own right. - Try to use a mixture of root position and first inversion chords for good effect. Do not have too many repeated notes in the bass.
In V-I and V-VI, the leading note (3rd of V) should rise to the tonic (1st of I, or 3rd of VI).
With V-I in a minor key, the leading note may rise a (diminished) 4th to the (minor) 3rd of I.
Dissonances
Most dissonances resolve by step to the nearest diatonic note. - Usually they resolve downwards.
For example, all 7ths and suspensions resolve downwards by step. (By 'step' I mean to the diatonic note immediately below, whether this is a tone or semitone makes no difference).
Avoid false relations (also called 'cross-relations' in America). This is when two different chromatic spellings of a note occur in different parts. (For example, F and F#, or B and Bb). - Where applicable, try to have the chromatic movement in the same part instead.
Motion between parts
There are 3 basic types of movement between parts. Using a mixture of all of them generally leads to best results:
Similar Motion
This is where parts move in the same direction, either both going up, or both going down - the intervals between the parts do not have to remain constant. This is ok, but too much similar motion quickly becomes boring, so avoid using it for too long.
Where two parts move in similar motion AND the intervals between them remain constant, they are said to move in parallel motion.
Parallel 3rd and 6ths sound good, and should be used when possible.
Parallel 4ths must NOT occur between the bass and any other part. (parallel 4ths are acceptable with the top parts, but should always be used sparingly).
Parallel 5ths and Parallel Octaves must NEVER, under any circumstances, be used between any two parts. - Though this only counts between perfect 5ths and perfect octaves. - If there is only one rule you ever learn, it should be this one. Everything else could be fantastic, but if you have parallel 5ths or octaves, your music will be ruined.
Note though, there is a difference between parallel octaves (which occur as an accidental by-product of part-writing), and doubling a part at the octave (to strengthen it, or for a particular effect). - The latter occurs often in classical music, but the former rarely sounds good in any style of music.
Contrary Motion
This is where parts move in opposite directions. - One goes up while the other goes down. This is the most effective type of motion between parts, and usually sounds very good. Try to have as much contrary motion as possible, especially between soprano and bass.
Oblique Motion
This is where only one part moves, while everything else stays at the same pitch. This gets boring very quickly though, so use sparingly.
Exposed Octaves and 5ths
This is where the soprano and bass approach a perfect 5th or perfect octave in similar motion. This should always be avoided, except when the soprano moves by step, in which case it is permissible.
Here are examples of most of the above points:
As always, any questions, let me know.
