Gudok is an ancient Russian string musical instrument, which was played with a bow. Gudok usually has 3 strings: 2 of them were tuned in unison and the 3d - one fifth higher. All 3 strings were placed in the same plane, so a bow could make them all sound simultaneously (not like in violin, where only 2 strings can be reached at any momement). Sometimes it has also several several (up to 8) resonant strings under the upper sounding board. These made gudok's sound warm and rich. When playing gudok an artist held it on it's lap like a cello or viola da gamba. Initially (in 12th century and probably before) gudok was played using harmonics only, without pressing strings to instrument's neck. Later (in 14th century and after) some modifications of gudok had a real neck for pressing strings, but this wasn't very useful because of specific instrument's shape and was an obvious borrowing from fiddles.
Now gudoks are entirely ceased away for several centures already. Nobody really plays them and only in museums you can see some reconstructions. Gudok always have been a folk, skomorokh instrument, so very few of them remains to our time.
There were several attempts to revive gudoks in music. In Borodin's opera Prince Igor there's a 'Gudok Players Song', which is an authour's reconstruction of how gudok may have sounded. But in practice it's performed with modern violins and altos of course.
Nowadays some folk Russian groups try to make their playalble gudok reconstructions and to use them on their performances.