Jim Lad said (before we were so kindly advised where to find some dubious thrills)

["You simply HAVE to tune on stage, all the time. People should be educated to understand that." Nope. Sorry. If you can't play the instrument without knocking it out of tune, you need a better instrument.]

I was actually the guest to whom he was replying (sorry, I must have forgotten to put my name).

Actually, Jim you are the one who is mistaken here. I play two top-end Martins and the others are all very good quality hand-made instruments (by David Oddy amongst others).

This has nothing whatsoever to do with knocking an instrument out of tune. It's to do with the physics of a vibrating string, the changes that take place within a wire when you relax or increase the tension, the effect of temperature and humidity on wood, the natural bendability of wood, and above all the compromises that have to be made to permit fixed fret positioning on instruments such as guitars.

You watch any top guitarist, from Ed Boyd to Vin Garbutt to Chris Newman to - yes - Martin Carthy, they retune constantly - because they HAVE to. And when they tuning to, say, Drop D, they have to keep retuning the whole guitar for a while before they can proceed.

Why? Well, if you drop the pitch of a string by one tone, it is a scientific inevitability that the metal in the string will 'spring back' slighly over the period of about one minute, to raise the pitch again by about half a semi-tone. So if you tune the string perfectly and then begin your song, it will go out of tune about half way through the first chorus.

And as the string relaxes, so the wood of the neck will bend in on that side. It's only a fraction of a mm, but it's enough to put all the other strings slightly out.

If you change two strings the effect will be greater (which is why I for one only ever retune one string at a time - carefully migrating from Standard to DADGAD and beyond through my set).

Then there are the issues of capo positioning. Both my Martins have perfect intonation. When I fret with my fingers the instruments are in tune right up to the very top fret. But capos - whichever make, they'll all do it once they're tight enough for use - pull the strings in tighter to the fretboard than any finger can. This always puts the whole instrument sharp by about 5% per fret.

So if you capo up - and need to stay at concert pitch - because you're playing with others, you must retune. Same when you take the capo off or move it down.

And you have to revisit each string more than once, because they are all changing in pitch by themselves, and the neck is constantly twisting as a result.

Now you may think it doesn't matter if the instrument is not perfectly balanced, and you may be right. Certainly one can become unhealthily obsessed with tuning. But when I say that a badly tuned instrument doesn't ring properly I'm not just referring to it sounding out of tune to the audience, or even to the guitarist.

I'm talking about its temper: The way the overtones vibrate against eachother. If you get this right the guitar suddenly comes alive in your hands, and playing becomes easier and a lot more fun. If the temper is out of sorts you can still play all the same notes, but it's hard work and the magic simply can't happen.

You must have noticed this yourself in your own playing.

I'm all for telling people that tuning is more complicated than it looks (and sounds), and generally educating people about it, as it really is worth making the effort to get it right - and to go on making the effort thoughout the gig.

Most people think a musician is just being incompetant if he can't seem to tune all his strings with one tweak to each, and that's a shame, because it's not easy to achieve good temper - specially while entertaining an audience - and it would be nice if people like Martin Carthy got the credit for taking the trouble to do it well.