The most common material for the highest (treble) courses. Sometimes used for the third course. Most often transparent, but also available in black or red. Plain nylon is often glossy in appearance, whereas rectified nylon is finished to a matte sheen for increased grip when plucking with the fingers (as in classical guitar or lute). Either is used for oud.

Nylon Multifilament (floss) Silver wound
Used for the lower courses

PVF (fluorocarbon)
Plain, unwound strings that have a higher density than nylon. At the same tuning and scale, PVF strings will be thinner gauge and brighter sounding than their nylon conterpart.

A sythetic "gut" material, milky-white in appearance. Also denser than nylon, but with a warm sweet sound. Used for the two upper courses. Made exclusively by Aquilacorde.

Nylgut multifilament Silver and Copper wound
Used for the lower courses, wound with either silver or copper wire. Made exclusively by Aquilacorde.