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Guitar Timer
The primary purpose of a metronome is to approximate rate of speed or tempo. It is a good tool to help you increase your speed for playing scales and arpeggios. 

I recommend that once you have speed and an understanding of playing between the beats, practice without the metronome. The reason for this is music is fluid and should come from your heart and not sound mechanical. Some of the best music speeds up and slows down with feeling from emotions and or feeding off of a live audience. 

In the studio a click track can get you started with the right timing but once the song starts the click track should end and feeling should take over. A click track makes it easy for the recording engineer but makes bad music in the end. A good drummer with soul beats a drum track any day.
The Beat Goes On - Each beat is approximately 10 minutes long.
When playing between beats, accentuating the first beat of each measure is helpful in keeping count.
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How to use a Metronome

Use a metronome to improve your timing and speed. Play on the beat and play between the beats.

Playing on the beat

Play a note on your instrument for every beat of the metronome. Slowly increase the tempo as you play until you find the tempo that you have trouble keeping up with. Play at this tempo until you can keep up with it. 

Playing between the beat

Playing 'between the beat' means playing a note at some time other than the start of a beat. If a bar has 8 eighth notes in 4/4 time, the first note is played 'on the beat and the second is between the beat, the third is on the beat, and so on.