The viola da gamba (also called the "viol" or "gamba") is not a fretted cello!
It may look like one, but a cello has 4 strings and a viol usually has 6, like a guitar, or 7. In addition, the viol's frets aren't permanently set, like those of a guitar, but are instead made of gut tied onto the neck, like those of a lute, and are therefore movable.
Viols are bowed, like cellos, but the bow is held differently-not overhand, as is a violin or cello bow, but underhand, like a pencil or chopsticks.
Viols are also tuned differently than are cellos. Cellos (and violins and violas) are tuned in fifths. Viols are tuned in fourths, with a third between the third and fourth strings, just like a lute and almost like a guitar. Chords can easily be played on the viol with the bow and are often included in solo music.
Like the cello, the bass viola da gamba is part of a family. The smallest, highest-sounding member is a treble viol, equivalent to the violin. Next larger and deeper in tone is the tenor viol, approximately equivalent to the viola. Even larger and deeper-sounding is the bass viol, equivalent to the cello. The largest, deepest size, the double bass, is the only viol played in orchestras today.
Viols have a long history. They were perhaps most popular in the 15th to 18th centuries, from about the time of Henry VIII of England, who played them, to that of Louis XIV of France (the Sun King). Shakespeare mentions them in several plays, including Twelfth Night.
The sound of the viol is sweet and shimmering, quieter than that of violins, violas, or cellos. Viols smaller than double basses are, in fact, too quiet to be effective in large orchestras or big concert halls, which is why they are no longer very common. But many people today love the particular timbre of viols and the Renaissance and Baroque music written for them. Concerts are usually given in small halls or churches, which suit viols well.
Composers for the viol include J.S. Bach , Marin Marais, Henry Purcell, François Couperin, William Byrd, and Orlando Gibbons. You can find recordings with viols by Jordi Savall, Fretwork, Phantasm, the Rose Consort, Weiland Kuijken, Sarah Cunningham, Paolo Pandolfo, Sex Chordae, Jonathan Dunford, Les Voix Humaines, Musical Assembly, and the New York Consort of Viols, among others. Look for them in the Early Music section of your local and virtual record stores.