From A Geometry of Music, Dmitri Tymoczko:
"Any two major chords can be connected by stepwise voice leading in which no voice moves by more than two semitones. This means you can write a harmonic progression without worrying about melody; that is, for any sequence of major chords, there is always some way to connect the notes so as to form stepwise melodies.
What about the chromatic cluster B, C, Db followed by E, F, Gb (its transposition by ascending fourth)? Here, none of the notes of the first 'chord' are within two semitones of any note in the second, and hence there is no way to combine a sequence of these chords so as to produce conjunct melodies. At the same time, however, the chromatic cluster can do things that the C major chord can't. It is possible to write contrapuntal music in which individual melodic lines move short distances within a single, unchanging harmony. This is possible only because the chord's notes are all clustered together, ensuring that there is always a short path between any two of them."
"Any two major chords can be connected by stepwise voice leading in which no voice moves by more than two semitones. This means you can write a harmonic progression without worrying about melody; that is, for any sequence of major chords, there is always some way to connect the notes so as to form stepwise melodies.
What about the chromatic cluster B, C, Db followed by E, F, Gb (its transposition by ascending fourth)? Here, none of the notes of the first 'chord' are within two semitones of any note in the second, and hence there is no way to combine a sequence of these chords so as to produce conjunct melodies. At the same time, however, the chromatic cluster can do things that the C major chord can't. It is possible to write contrapuntal music in which individual melodic lines move short distances within a single, unchanging harmony. This is possible only because the chord's notes are all clustered together, ensuring that there is always a short path between any two of them."