Diabolus in Musica Guide to Early Instruments

Bowed strings include viols, violins, rebecs, and hurdy-gurdies. In these instruments, the sound is made by a bow being drawn across the strings, or rotated against them in the case of the hurdy-gurdy.

Plucked strings are instruments in which the strings are plucked either with the fingers or with a quill (the tip of a feather). This group may be further divided into :

Fretted instruments like lutes, citterns and guitars, in which one string or course is used to make several different notes by pressing it down onto successive frets, thereby changing its effective length.

Unfretted instruments like the harp, which use open strings - ie, each string plays only one note, and its length is not changed. Note that violins are sometimes referred to as "unfretted". It's true that they don't have actual frets, but the principle of altering the note by changing string length is used in the same way as on viols.

Most Keyboard Instruments use strings to make their sound. They are either plucked (harpsichords), hammered (clavichords), or bowed (Geigenwerks). The exception, of course, is the organ, which is blown, and doesn't use strings, except to hold bits together when it's falling apart.

Stringed instruments in the medieval and renaissance periods used two common string materials. The first, and by far the most common, was sheep gut - the intestines of sheep, washed and spun into tough filaments which still make the best sounding strings, despite modern advances in materials science. It's very difficult, unfortunately, to fully reconcile renaissance performance practices with vegetarianism. That's one of the dilemmas of modern life.

Incidentally, the common name "catgut" is a misnomer, or at least, it's nothing to do with cats. "Catline" seems to have been a term both for the lower strings of viols and for certain ropes on sailing ships of the period, so it was probably originally a nautical term. Another theory is that it's short for "catapult line", the springs of siege catapults being normally made of huge quantities of gut. The other common string material was metal, either iron or brass. The main metal-strung instruments were harpsichords, citterns, psalteries and dulcimers.

Keyboard instruments and plucked string instruments, particularly in the sixteenth and seventeenth centuries, often had their strings doubled, ie. two strings ran side by side, they were tuned to the same note, and plucked as one string. This gave a louder, fuller sound. The general name for a group of strings which act as one is a "course". A typical renaissance lute, for instance, had seven courses. The top course was single (ie. a single string, which allowed fast playing). The other courses were double-strung. So, a "seven course" lute plays like a seven-stringed instrument, but actually has thirteen strings.

I said above that the two strings of a double-strung course were tuned to the same note. Usually they were, but an even fatter sound, with more harmonics, was sometimes achieved by tuning them an octave apart. Renaissance lutes often had octave seventh courses. Probably the oddest arrangement was on the renaissance four-course cittern, which was strung in wire. Here, the first, second and fourth courses were unisons. The third course was triple-strung, with two strings tuned to a unison and a third to the octave. Furthermore, the fourth course was higher in pitch than the third - this is called "Re-entrant" tuning. Weird, but it works, and sounds wonderful.