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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