MIMF

I have been asked to replace the fret(s) on a Mexican vihuela owned by our school district and used in their mariachi band. It has five nylon frets - one has come untied and there is not enough material to retie it (they are apparently cut very close to the knot after tying).

The present frets measure about 0.55mm (0.020 inch) in diameter and are a light blue color - could very well be some sort of fishing line? They pass three times over the fingerboard and have loop on the back that would allow the line to be pulled tight before tying. When putting the new one on I will attempt to duplicate this arrangement.

So, my questions are - what is the best (or traditional) material to use? How do I tie the knots at the end so they don't slip or loosen? Where on the back of the neck should the knots be located (now two of them are towards the center of the back of the neck, the others are offset to the bass side)? Unless I can perfectly match the diameter of the existing frets I assume I should replace all of them - any trick for checking the action afterwards?

As always, thanks.

The traditional material is gut. However, this has not been much used ever since the advent of nylon. So nylon it is, more than likely fishing line, as you say. Lutes have been (and are) equipped with tied-on frets ever since they have been in use. Go to Google images, and tap in "tying lute frets". There are a numbe4r of instructions available there.
For that matter, tap in "tying saz frets" as well. They do it differently, having more winding around the neck, sounds more like the vihuela you have.

Bingo, Yuri, thanks. All of the articles on lute frets show one or two wraps but saz apparently have three with a similar tensioning knot. I'm off to the sporting good store with my calipers to buy some fishing line, will report back.

This reminds me of a line from Shakespeare's "Hamlet". Hamlet's friends Rosencranz and Guildenstern have been pestering him to do them a favour, and he refuses with the words, "Though you may fret me, you cannot play on me!" Obviously a pun on "to fret", meaning to annoy or to tie a fret round a lute neck, extended to "play on", meaning to make music on an instrument or to manipulate a person.

If we take the figure of speech as pedantically as the Elizabethans often used their puns, we could deduce that you have to fret a lute (i.e. tie the frets) before you can play it; even more pedantically, it could be that the first lesson one learnt on a lute was to fret it, and only when you could do that, did you learn to play. That is, tying frets was perhaps even considered a basic skill for lutenists back then.
Fretting entails not only tying knots, but also placing the frets so as to get correct intonation - so there wouldn't have been much point in practising scales and chords before you had masterd it!

Cheers,
John