Tic-tac-toe squares, e.g

• Music for a baseball team?
• Nine-piece combo
• Musical composition — tonne (anag)
• Quartet plus quintet
• Trio, tripled
• The Muses, for example
• Certain string ensemble
• Large musical combo
• Work for nine players

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