Proper maintenance of your guitar includes knowing when it’s time to replace your guitar strings. When the high strings on your guitar turn black and rusty you have to change them. When the low strings on your guitar (the fatter strings) get dents in them from the frets, they must be changed. The guitar strings must be replaced because a rusty guitar string or a dented guitar string is no longer round and will not play a true note. Your guitar will no longer play in tune.