Tic-tac-toe squares, e.g

• Tune for nine
• Multipart opus
• Group of nine performers
• Good-sized combo
• Score for a baseball team?
• The Muses, for example
• Northern town initially housing a single music group
• Work for nine players

Wiktionary Word definitions in Wiktionary
n. 1 (context music English) a composition written for nine instruments or nine voices 2 a group of nine nuclear or subatomic particles 3 (context computing English) A byte of nine bits.

Wikipedia Word definitions in Wikipedia
Franz Lachners Nonet in F major is a composition for chamber ensamble published in 1875.