Appart from the main note we hear from a string it contains a series of overtones. Due to inharmonicity the overtones are too high and the higher in the series of overtones we get the more too high they become. What we apprehend as a single note is rather a wider tone consisting of the main vibration plus all the inharmonic overtones. A new string contains a maximum of overtones.

When the string gets old the overtones will be muted one by one by dirt and corrosion. On wound strings the winding becomes deformed against the frets. We apprehend that the tone gets poorer. When he most inharmonic overtones disappear, the tone becomes more narrow and actually gets closer to the main note of the string. The tone actually gets purer (truer) and the flaws in intonation becomes more evident. New strings are not improving the intonation - they just hide the bad intonation of the instrument!

Nowadays there are expensive, fibre coated strings that resist dirt. This way bad intonation can be camouflaged longer. Yet a well intonated instrument will sound good even with old strings because the remaining, less inharmonic overtones will resonate easier. There is no problem tuning old strings as both intonation and tuning are performed to the main vibration of the string which is registered by the tuner. Also the human ear apprehends the main vibration in the first place. The main vibration is not altered by aging.

In the long run an intonation ought to pay off in terms of lower string costs - besides the increased value of the instrument and the invaluable satisfaction of playing on an instrument in tune. Very few musicians have enjoyed that on fretted instruments!