Here's some info from Wikipedia about this old instrument:
The Langeleik is a traditional stringed musical instrument from Norway. The instrument has one melody string, like a monochord, but it has additional bourdon strings (or drone strings), usually 7 or 8 on modern instruments, but older ones had fewer, most often four bourdon strings. The frets under the melody string are placed such that it can only play a major scale (around A major, however it is referred to C major for the ease). The bourdon strings are also tuned to the triad of the melody string ( in quasi C, E and G (really somewhere about A, C# and F#)). Since the instrument cannot play a chromatic scale nor be easily tuned to other pitches, it is very limited in its ability to play together with other instruments and/or more harmonic complex music. As a sidenote, the sound you get when you play other tones than those of the bourdons (the triad C-E-G), is quite special. The oldest known langeleik in existence is dated 1524. It is clearly related to other European instruments, but mostly to the German Scheitholt and next the French épinette.