A tuner is a device musicians use to detect pitch accuracy. It will let a musician know if the note they are playing is sharp (too high), flat (too low), or if it is in tune. The accuracy of a pitch is what musicians call intonation. Tuners work by detecting the frequency of the pitch (sound waves). For example, an A is 440 Hz. If an A is sharp, it will be 441 Hz or higher. If it is flat, it will register as 439 Hz or lower. While tuners work by tracking hertz, musicians measure how close they are to the pitch in measurements of cents. Cents and hertz are not the same things.