A Metronome is a piece of equipment that produces and audible and/or visual rhythmic beat. Most will produce a tuning note at 440hz. It is used to stay in time with music time signatures and to help practicing difficult pieces slowly and train up to full speed. The time of a certain music composition is called the Tempo. Tempo is usually measured in BPM (beats per minute).
Why use a Metronome?
A metronome helps keep the established time of a piece of music. Without one, the playing would be most likely out of time with other parts of the music. A metronome can help you practice very difficult parts of songs at a slow pace until you are comfortable to move up and up all the way to full tempo. A metronome will help you achieve professional speeds in your playing.
One common type of metronome is the wind-up metronome, which uses an adjustable weight on the end of a rod to control the tempo. Slide the weight up the rod to decrease tempo, or down the rod to increase tempo. The pendulum rod swings back and forth in tempo; mechanics inside the metronome produce a clicking sound on each swing of the rod
This kind of metronome is used very commonly and can provide several different beeps and clicks for musicians to follow.