Spirocore strings have a flexible multiwire spiral rope core. They offer a longer period of musical vibration and significantly reduce inertia. Spirocore strings are equally effective when playing arco or pizzicato. They are highly responsive and long lasting. Spirocore strings, widely recognized as the reference standard are renowned for their distinctive tone and playability.This extra long fourth string is made to tune to low D on an extension, which is a step higher than the long orchestra tuning string.