A guitar Tuner (tuner) is a device that is used to tell if a certain string has the correct amount of tension to make its desired tone. Tuners  come in digital, analog or non-electric forms. Analog guitar tuners use a needle to indicate whether a string is flat or sharp and has a midpoint line that indicates that the string is in tune. Digital guitar tuners use either a digital version of the analog tuner system or a series of lights that move to the left if the string is flat, and to the left if the string is sharply out of tune.