The metronome is one of the most important musical inventions of the past few centuries. It not only allows musicians to keep track of the tempo of a piece without worrying about it, but it also helps to standardize tempo labels for performances. The modern day electronic, and even Internet, metronomes are a big improvement on their mechanical cousins, but their purpose as a practice aid remains the same.
The main purpose of a metronome is to keep time by clicking at a given tempo in order to aid musicians in their practicing. Metronomes can be adjusted so that they click very slow all the way to very fast tempos.
Classical musicians most often practice with the metronome on all beats of a bar, or on beats one and three. They will also sometimes put the metronome on the eighth note in particularly difficult or fast passages.
Jazz musicians often practice by setting the metronome on beats two and four. This helps them to work on their "swing" feel, which is essential when playing jazz music.
Recording studios use electronic metronomes called "click tracks." Click tracks are used to make sure that the tempo of a song stays the same when recording instruments, or vocalists, separately.
The mechanical metronome was invented in 1812. Beethoven was the first composer to put metronome tempo markings in his compositions.
There are two main types of metronomes, electronic and mechanical. Mechanical metronomes are crank operated, while electronic metronomes are battery powered.