This geometric term is called a musical instrument, which is included in shock and quite often used in symphonic and operatic music. The shape of the tool is an equilateral triangle. It is made from steel rod. The triangle is fixed to the panel and gently hit the metal rod. The sound is high (unspecified height), clear and soft, and the strong impact poignant, reminiscent of the bells.
In Grieg's music to the drama of Ibsen's "peer Gynt" triangle introduced in Anitra's dance.
Edvard Grieg. Anitra's Dance
His ringing trill emphasizes the elegant, whimsical nature of the dance. And in "Scheherazade" and "Spanish Capriccio" by Rimsky-Korsakov rhythmic tinkle of the triangle gives the music even more sparkle, liveliness, enthusiasm.