A springald, or espringal, was a medieval torsion artillery device for throwing bolts. It is depicted in a diagram in an 11th-century Byzantine manuscript, but in Western Europe is more evident in the late 12th century and early 13th century. It was constructed on the same principles as an Ancient Greek or Roman ballista, but with inward swinging arms and threw bolts instead of stones. It was also known as a 'skein-bow', and was a torsion device using twisted skeins of silk or sinew to power two bow-arms.