مَثْلَثُ

and مَثْلَثَ: see ثُلَاثُ.

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.)