Mean tone dulcimer?
General mountain dulcimer or music discussions
James, the conversation that Strumelia links to will indeed help you understand the difference between mean tone and equal temperament as they pertain to the dulcimer, and Peter's explanation also offers more than I can.
But let me just explain the problem that these two fretting systems try to address. It is sometimes said that G-d invented the octave and the fifth and humans invented all the other notes. Imagine a string tightened over a soundbox with no frets. Obviously, if you stop the vibration exactly half way along the string, you will get an octave of the open string. But between that open string and the octave, how many notes should there be? Different cultures answer that question differently. Some have 5 notes, for example. In western music, a chromatic scale has 12 notes and a diatonic scale has 7 notes. But where along that open string would you put them? It turns out that placing those notes along the string is not as simple as it seems, and exactly where they go would actually depend upon the note they are tuned in relation to. That is why in John's discussion Robin talks about placing frets not in relation to the open string, but in relation to the fretted tonic note.
So if you were fretting an instrument to be played in only one key, you could do a great job of placing those frets so that the notes would all sound great in relation to one another. But if you tried to play in a different key, they would sound off. Equal temperament tuning tries to basically split the difference and create note placements that are equally off in all keys, but hopefully close enough so that most of us can't hear the dissonance.
Here is a mathematical explanation of all of this: http://mathforum.org/library/drmath/view/52470.html . The first response by Dr. Toby might be all you need, but the conversation there goes on for some length.

Do you have a choice of 'keys' ? This is a truth; I used to play in a pub session wherea