On the fourth beat of the first measure, the string instruments play double-stop harmonics from the highest possible notes of indicated string. This action is indicated with a texted explanation (Double stop harmonics…) and graphically notated notes consisting of arrow notes (indicating highest notes) and diamond-shaped notes (indicating harmonics). Since both of these signs, the arrow, and triangle-shaped notes have now become part of the standard notational library the texted explanation is not necessary. The composer has chosen to emphasize the technique as the continuation is somewhat special.
The open triangle appearing above the high double-stop harmonics of the string instruments indicates what is usually named “col legno” in standard music writing, or use the wood of the bow. There are two styles of col legno tratto and battuto, the first one meaning strike the wood of the bow over the strings, the other one to hit the strings with the wood of the bow. So why not use the standard notation of writing “col legno tratto” instead of the open triangle sign? Since using three different types of bowing, hair, wood, and combined hair and wood, all for striking the bow in vertical motion instead of the normal horizontal, it somehow felt right to use triangle sings. Open triangle for wood on strings, black filled triangle for wood on strings and half-open half-filled triangle for the hair-wood combination. That way it was clear that the bow should at all times strike the strings (tratto) not hit them (battuto) and the triangle fills easily to understand.