A block chord is a chord or voicing built directly below the melody either on the strong beats or to create a four-part harmonized melody line in "locked-hands" rhythmic unison with the melody, as opposed to broken chords. This latter style, known as Shearing voicing (#Voicings), was popularized by George Shearing but originated with Phil Moore.
Block chord style (also chorale style) uses simple chordal harmony in which "the notes of each chord may be played all at once" as opposed to being "played one at a time (broken or arpeggiated chords). For example, a person playing a guitar can strum the chord (this would be a "block" chord) or use a picking style to play "broken" chords". The notes of arpeggios are often grouped into block chords for ease of analysis.
Block chords and doubled melody are easily used in a melody line that has a swing feel and strengthen the melody so as to separate that melody from the rhythmic background. Block chording was used to a large extent by jazz bands and orchestras such as those led by Count Basie and Duke Ellington.
In addition to George Shearing, Red Garland was an early jazz pianist famous for his use of block chords. Garland would play 7 to 8 note voicings, often playing a non-moving chord in his left hand, then an octave in his right hand, with 1-2 notes in between. Fine examples of this can be heard on various recordings of his time with the Miles Davis quintet. Bill Evans is also remembered for his use of block chords when he played in Miles Davis' band in 1958.
"A common way to harmonize tunes 'as you go along' in jazz piano (ie, freely and flexibly) is known as block chords: the hands move in parallel, providing a chord for each note of the melody. This often uses a technique derived from the way jazz arrangers write for four horns ('horns' in jazz means saxophones, not the brass instruments of classical music) or four trumpets: this is called four-way close."
There are a variety of voicings or methods:
- Generic block chord describes those that simply follow the above rule.
- Double melody (Commonly called the "Shearing voicing") with an additional fifth part that doubles the melody an octave lower.
- Drop 2 (technically not a block chord) with the second voice from the top transposed one octave lower.
If the melody note is part of the chord, the harmony notes are also taken from the chord.
This is a good technique if the melody note is diatonic (and not chromatic) and uses diminished chords for the notes that are not part of the chord. If the melody note is considered a passing tone, the harmony is created either by a diminished chord or a chromatically shifted chord. Before creating the harmonies, the chords could be converted to 6th chords, but this is not a rule.
The following is an example of harmonization of a C major scale with block chords. This example uses three diminished chords on the D, F, and B notes and includes an additional diminished chord on G#. This creates a balance in the harmonization of this scale by using all four existing diminished chords.
The "Shearing voicing" is described as being achieved through playing the melody in both hands, playing the appropriate chord below the right hand melody note and bringing out the melody with the left. (Courtesy of Wikipedia)