Instruments of the Viola da Gamba family, often called viols, are made in very many shapes and sizes. The most common sizes of viols are the treble, which is about the size of a violin, the bass, which is about the size of a cello, and the tenor which is between the two. There are also larger and smaller viols. Some have a shape which resembles a violin, others have plainer corners or a much fancier outline. They are always held on the knees or between the legs and are usually played with a bow (which is held underhand). Most viols have C-shaped soundholes and a flat back. Strings are usually made of gut, like the frets which are tied around the neck and are partly responsible for viols' sound quality. The rich, clear sound of viols means they blend well with each other and with other instruments or voices. Most viols have six strings, but some have more and others fewer. Many tunings have been used but by far the most common has the following intervals between adjacent strings: 4th - 4th - 3rd - 4th - 4th, with the top string tuned to d, a or g.