The Viol dates back to the 16th Century. Like a violin, it produces sound when the strings are vibrated, often by the bow sliding across them. The body of the instrument amplifies the sound, but because it is flatter than a violin, the sounds are softer and more subtle.
The Viola Alto is a large five-string instrument developed in the 1800s. Its low tones resonates through its larger body.
The Viola d'amore was created in the 1700s, with seven strings and seven more "sympathetic" strings that create a rich harmony as they vibrate.