Bradley also spoke about other issues involving the instrument’s flexibility, including the use of flexible horn plectra instead of wood, and the introduction of the 5th string by Ziryab, early in the 9th century. Ziryab was booted out of his home town Baghdad by a teacher who was jealous of his talent. He settled in Cordoba, in the kingdom of Andaluz. According to Bradley, he was a man of elegance and sophistication, and is responsible for promoting glass drinking vessels and is supposed to have been the first man to eat asparagus. (http://userpage.fu-berlin.de/~telehist/ambass/ud.htm) In Cordoba he got himself in trouble with the ruler, and had to leave again (in 821/22), and this time he went to Spain, where he was celebrated, and started to really concentrate on his life’s work, (Hickmann, 1970: p. 23-24). Tradition states that he added the fifth string to the ‘ud, though it is not certain, whether at the top or bottom of the instrument.

The lute not only played beautiful music, but also symbolized something for the people who listened to its music or played on its strings.
The four strings symbolized the elements, the phases of the moon, the directions, the seasons, the weeks of a month, the divisions of a day, of a body, of human life, and the four humors – that is, in descending order, the yellow bile, the blood, the phlegm, and the black bile. They were tuned a fourth apart and, according to the theorists of the 10th century, they were twisted from threads, the number of which decreased in the proportion 3:4; from bam to zir they consisted of 64, 48, 36 and 27 threads respectively. (Sachs, 1940: p. 253)

(Polin, 1954: plate XXIV)

In the 9th century, Mawardi, the ‘jurist of Baghdad’, praised the ‘ud for its use in treating illnesses. This principle was allowed and defended in Spain in the 11th century by the theologian Ibn Hazm. Still in the 19th century, Muhammad Shihab al-Din writes about the ‘ud’s power:
‘the ud invigorates the body. It places the temperament in equilibrium. It is a remedy … It calms and revives hearts’ (Muhammad Shihab al-Din: Safinat al-mulk, p. 466) …
(http://www.kairarecords.com/oudpage/Oud.htm)