They sing ¹  to the tune of ²  stringed instruments; ³  like David they invent ⁴  musical instruments.

Ahaz was twenty years old when he began to reign, and he reigned for sixteen years in Jerusalem. ¹  He did not do what pleased the Lord, in contrast to his ancestor David. ² 

On that day the Lord himself will defend the inhabitants of Jerusalem, so that the weakest among them will be like mighty David, and the dynasty of David will be like God, like the angel of the Lord before them. ¹