if its a classical guitar, u shudnt detune all the strings too much simultaneosly, as overdetuning (yes its a made up word) can cause the top of the guitar to lose tension and could result in cracks forming in the wood. for the same reason, strings shud be changed one at a time.
strings can also lose sonority over time due to hand perspiration. this can b rectified either by replacing the string in reverse (classical), buying new strings or washing the string in soapy water and then replacing it when properly dry (dunno if tis works for electric guitars)
@guitarangela: if the strings develop a sudden buzz, it is possible that the winding ofthe string has lost som of its original tension, or the groove in the nut has deepened. two possible remedies r to change the string or to place a small thin piece of cardboard below the nut.
similar sort of problems can develop as the result of exposure to varying humidities (classical guitars).
i know that most of the people in this thread probably hav electric guitars, but some of wht i hav written can apply to electric guitars aswell as primarily applying to clasical guitars
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