Pirastro Obligato bass D string, chrome-steel wound over a synthetic core. Orchestra (medium, mittel) gauge. Catalog #442120. Package design may vary slightly from what is pictured here., Obligato strings have a core made from a modern synthetic multifilament fiber. They sound warm and full, yet have a brilliant, rich overtone spectrum and focused tone. They feature excellent playability and response throughout the entire dynamic range. In the Metzler Violin Shop we use Obligato strings when a darker, richer tone is required.