When the bow moves over the strings, the strings generate primary vibrations that are communicated to the  body through the bridge and amplified by the body. Originally, strings were made of catgut. As this fiber is not very durable and susceptible to temperature and humidity changes, steel strings became popular especially for the E-string of violins from the early 20th century. Catgut wound with metal wire remains in use for the three lower strings, but synthetics have become the material of choice for the core of strings.