ثلث

مَثْلَثُ مثلث and مَثْلَثَ: see ثُلَاثُ. ― -b2- مثلث [i. e. مَتْلَثٌ] signifies A chord [ of a lute ] composed of three twists: that which is of two twists is called مثنى [i. e. مَثْنًى]: or, as some say, these two words signify [respectively] the third chord and the second: their pls. are مَثَالِثُ and مَثَانٍ. (Har p.244.)