Some Notes on Harmony
Traditional Treatment of Harmony in Arabic Compositions
As a general rule Arabic music is monophonic. Whether a small ensemble (takht) or a large orchestra are playing, all instruments will play in unison or in octaves. In this sense the larger orchestras do a disservice to the music because they drown out the softer instruments (such as the oud). There is also little room in large orchestras for the nuances and embellishments of individual musicians that are so common in Arabic music and that are such a fundamental characteristic of its aesthetic. There are some interesting exceptions to this monophony.
Contrabass and qanun.
Qanun solos would sometimes use parallel motion (mostly diatonic thirds) or pedal tones that imply harmony. Of course, parallel octaves which a lot of Arab qanunists use is not considered of harmonic nature, it simply adds richness to the melodic line.
Cellos and contrabass are both used in larger orchestras. While the cello line usually follows the melody, contrabass lines often don't. They outline the important notes in a melody.
Arabic pop music
Modern pop Arabic music which makes limited use of harmony. This appears typically as a sporadic use of chords executed on keyboard or electric guitar, usually not in any form of functional harmony, though the occasional V-I business does take place.
Chords used as coloring
Another interesting exception appears in some compositions of Abdel-Wahab and Baligh Hamdi, among others. Again, this would appear as a sporadic use of chords executed by keyboards or electric guitars in the instrumental parts of some songs. And again not in the form of functional harmony.
Compositions employing western harmony
Finally, in some modern compositions harmony might appear as an integral part of the composition and not as coloring. This is the case in Marcel Khlife's "Jadal" an instrumental composition in four movements. For example polyphonic lines in parallel thirds appear suggestive of Greek folk music.
Orchestral and choral arrangements of Arabic melodies
Khalife and others have also written orchestral and choral arrangements of traditional Arabic melodies that employ harmony. While a bit enjoyable occasionally, these arrangements and compositions, to our ear, do not have an Arabic music feel to them.
The musician must be mindful that if harmony is used in an Arabic music context, ornaments and embellishments (that would traditionally be used without hesitation) should be well considered and sometimes need to be avoided as they create strong dissonances when in a harmonic context.
Playing Chords on Oud
Only a limited number of chords in a very limited number voicings can be played effectively on oud. One limitation is that each note in the chord should be fingered separately (as opposed to laying down the first finger like a guitarist would do). The reason is that in the absence of frets, the soft part of the finger would absorb the string vibration virtually eliminating the oud's resonance.
The oud may successfully arpeggiate chords. Here again, some arpeggios are easier than others to play. The more difficult ones typically require some acrobatics to produce all the chord tones. Since the moment a note is not being fingered it stops resonating (unless it is an octave or two from an open string in which case sympathetic vibrations on the relevant open string remain), chords arpeggiated this way will sound one note at a time without residues of previous notes.