Комментарий к Берешит 4:21

ושם אחיו...אבי כל תפש כנור ועוגב, he was the first man to invent music and musical instruments. The reason why the Torah uses the term תפש, “holding in one’s hand,” is probably because the instruments such as guitars and violins have elongated parts which are held by hand.

ושם אחיו יובל, “his brother’s name was Yuval.” From the manner in which the Torah reports their births it is clear that they were twins. If not, his birth would have been introduced with: ותלד עוד, “she gave birth also, etc.” It is customary for shepherds to entertain themselves by playing the flute or a similar musical instrument. Most likely Naamah, Tuval Kayin’s sister, (verse 22) born by Tzilah, was also a twin, as Tzilah is not credited with having given birth twice.