Violinists generally carry replacement sets of strings to have a spare available in case one breaks. Even before breaking, worn strings may begin to sound tired and dull and become "false" over time, producing an unreliable pitch. Another common problem with strings is unravelling of the metal winding. Strings may need replacement every two or three months with frequent use. The higher strings require replacement more frequently than the lower strings since they are lighter in construction to produce a higher sound– fortunately their lighter weight also means they cost less. The price of strings varies, and the quality of the strings strongly influences the timbre of the sound produced. A teacher can advise students how often to change strings, as it depends on how much and seriously one plays.