Older nyckelharpa types
The earliest depiction in Sweden of something that is believed to be a nyckelharpa is from around 1350. The earliest instrument found, known to really have been played in tradition, is from the seventeenth century. The photo above shows a "kontrabasharpa" (photo: Magnus Holmström). You could say that all the older nyckelharpa types described below look quite similar at a cursory glance. Note for example that the sound-holes are round, so-called "ox-eyes", in stead of the f-holes you see on most nyckelharpas of today.
"Enkelharpa" is the nyckelharpa of the 17th century you could say, if you generalize somewhat. "Enkel" means simple, and refers to the fact that is has only one melody string (a1), a number of resonance strings (6 -- 13) and one or two bass strings (drone strings, i.e. strings without keyes, and of which, followingly, you cannot change the pitch).
"Kontrabasharpa" is, roughly, the nyckelharpa of the 18th century. It has two melody strings, tuned a1 and d1, a number of resonance strings (6 -- 13 st.) and one bass string. The melody strings are on either side of the bass string, hence the name ("contra bass").
You could say, roughly, that "silverbasharpa" is the nyckelharpa of the 19th century. It has two melody strings, tuned a1 and c1, 9 -- 10 resonance strings and one or two bass strings.
Kontrabasharpa med dubbellek
"Kontrabasharpa med dubbellek" (kontrabasharpa with a couble keyboard) is like a combination of kontrabasharpa and silverbasharpa. This type existed locally in Österbybruk, and is, accordingly, also called Österby-harpa. It has an a1-sträng and both a c1- and a d1-sträng. Furthermore it has 9 -- 10 resonance strings and one or two bass strings.
The contemporary nyckelharpa
The chromatic nyckelharpa with three rows of keys (see photo above) is the common nyckelharpa type today. This type can be said to have been developed by August Bohlin and Eric Sahlström, separately, around the 1920's and 30's, even if there are earlier attempts. When you, in everday speech, say "nyckelharpa" it is this type that you have in mind. This instrument has three melody strings: a1, c1 and g. There is one bass string, tuned c, and 12 resonance strings, one for each note of the chromatic scale.
Listen to the sound of me playing a traditional nyckelharpa tune - Masbopolketten ("The Masbo polka") after the Sahlström family. "After" somebody means that the person had it in his or her repertoire, not that he or she made it.