Although it's not a exact science on why guitar strings break, there are some simple reasons that will up the chances. Guitar Strings can break for any of the following reasons.
1. The string is over tensioned. Guitar strings are designed to be tuned to concert pitch or 440. They can be tuned a little higher but the risk of breaking the string increases.
2. If your strings are old they're more likely to break. Less elasticity, combined with wear and rust, caused by sweat on your fingers, will weaken guitar strings and eventually cause them to break.
3. Strings are often broken by heavy handed technique especially when strumming your strings to hard. This will cause your strings to break at any point in the length of the string. Lead guitar players seem to break less strings them rhythm players.
4. A kink in the string can cause a weak spot. Before a string breaks you can usually see and feel a little bump. Take care when changing your strings and make sure you don't bend them.
5. Sharp edges on the guitars saddle, nut or tuning head can cause strings to break. To fix the problem check where the string has broke and file down any sharp edges.
Hope this helps and maybe will save you some bucks on strings!
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