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It is my understanding that strings, over time, stretch and are unable to maintain their tension, so they go out of tune at a lower pitch.

When I pick up my guitar and I tune it with my electronic tuner, I often find most strings at a higher pitch than they should be... this means that they have gained tension... how is this possible?

This has happened to me with two different guitars in two different houses and over a sizeable period of time (months), so I'm starting to suspect that I'm doing something wrong.

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Good question. I have a guitar that has one string go up. Although, I always assumed that the rest of the guitar had gone flat. I've never even thought to check it with a tuner. –  yossarian Feb 8 '11 at 23:52
    
Does your guitar have a tremolo? Do you leave the tremolo bar on the guitar when it's in its case? Do you put anything over the neck or body in the case that could cause pressure against the guitar/neck when the lid is closed? Do the cases fit the guitar correctly? The guitar shouldn't shift or rattle around when the case is closed, nor should you have to force the lid closed. It should be a firm fit all the way around. –  Anonymous Feb 9 '11 at 3:00
    
@the Tin Man: this has happened to me with two different guitars, one with the whammy bar and one without. But, in both cases I tend to not put them in a case, but leave them on their support. –  UncleZeiv Feb 10 '11 at 23:12

4 Answers 4

up vote 9 down vote accepted

Your strings have gained tension due to the change in temperature and humidity causing the wood of your guitar to expand and contract. Metal strings don't expand or contract enough to cause detectable tuning issues alone. The wood of your guitar, however, does. The truss rod does the best job it can for keeping your neck at the right angle relative to the body, but it's not perfect. A slight movement of the neck backwards (towards the z axis relative to the body of the guitar) will cause your strings to go sharp enough for your ear to discern. You can test further this by playing open strings and pushing on the headstock in different directions perpendicular to the body of the guitar--just don't push too hard. As relative humidity changes, the wood of the body and neck of a guitar will expand and contract in two ways: tangentially and radially. Tangential movement lies parallel to the growth rings of the wood while radial movement is perpendicular across the growth rings. Another thing to consider is that different species of wood are more hygroscopic than others. This simply means that one species, take Maple with a radial growth coefficient of 0.00353 for example, will absorb more water--and thus move more--than another species like Cherry that has a lower radial growth coefficient of 0.00248. So, while the neck of your guitar gains or looses moisture, the wood will expand and contract relative to one of the two directions I explained earlier. This movement in either direction can very well be enough to cause more tension on the strings, thus bending them sharp.

A great article explaining all this a little more lives here. It's about cabinets, but very much applies to anything made of wood because the physical properties are universal.

Also, another list of some common woods and their growth dimension coefficients is here. According to that book mahogany is on the lower end of the spectrum, so it doesn't move very much. However maple, a very popular wood for guitar necks, tends to move a lot tangentially (parallel to the growth rings, or grain of the wood) and consequently is a little more susceptible to changes in humidity.

So there's the physics of it :D. In the end, the environment your instrument lives in is very much an factor to it's overall health--especially if it's made of real wood.

Also, see the answers on this question for other ideas as to why your guitar goes out of tune (usually).

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Ok, but why would only 1 of my 6 strings go sharp? –  yossarian Feb 9 '11 at 19:42
    
Context? Lots of reasons :D. –  Jduv Feb 9 '11 at 21:40
    
Thanks for the very thorough answer. It makes a lot of sense. I should have mentioned that this has happened to me both at my old place and at my current place, and I've had humidity issues in both of them. Plus, I'm a novice and I've got an entry level guitar, so I guess it's even more sensitive to the environment. –  UncleZeiv Feb 10 '11 at 23:15

This happens to me quite a lot, and I have put it down to the fact that I play my guitar in a number of different rooms in my house. The reason that this would have any effect is the room temperature.

If the guitar's surrounding atmosphere changes by even a couple of degrees, things can change. The metal strings expand or contract, depending on which direction the temperature is shifting. If the temperature gets cooler, the strings contract, which causes more tension, and vice versa. Changes in moisture and humidity can also cause differences, as the wood can change depending on humidity levels. Do you set your heating to go off at night or during the day? There could be many other reasons.

As with most tuning related problems, in my own personal experience the G and B strings are usually the worst culprits in this situation.

Hope this helps.

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When the humidity goes down (I live in Houston, so it can go down alot) all of my strings will be sharp. –  JohnOpincar Feb 9 '11 at 1:04
    
That makes complete sense. Lower humidity causes the wood of your guitar to shrink. Wood tends to shrink perpendicular to the grain, and necks are almost always cut with the grain running parallel to the fingerboard. Therefore, as the wood shrinks it will move backwards toward the body of the guitar. –  Jduv Feb 9 '11 at 1:31
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The strings actually don't expand and contract enough to cause detectable tuning issues as related to environment. They will expand and contract during usage due to forces exerted on them. –  Jduv Feb 9 '11 at 1:33
    
thanks, very nice answer Alistair, I wish I could accept two! –  UncleZeiv Feb 10 '11 at 23:13
    
Your welcome, and don't worry about it :) –  Ali Maxwell Feb 10 '11 at 23:23

If your guitar has a tremolo bar, it may also be caused by the springs that lose their tension (on a floyd rose type of tremolo, I don't know how the other systems work, but I guess it is also based on springs).

But I would go with the temperature or humidity change.

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On top of reasons such as humidity and temperature, It can also depend on how your strings are wound. Unwound strings in particular can slip if they're not wound correctly, these are the B and high E strings on most acoustic guitars. (For more information on how to wind strings, have a look at this question: How to wind strings on the tuning posts.)

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Although such slip would make it lower rather than higher –  jclozano Jan 2 '12 at 21:11

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