You've Probably heard the old Sonny and Cher song, The Beat Goes On.  And that's exactly what we're talking about . . . the beat.  And nothing can help a musician with that more than a metronome.  A metronome is one of those basic things that a studio or practice room should not be without!  No matter what your instrument, a metronome can be incredibly useful in helping a musician play or sing at the proper speed, maintain a steady speed and overcome rhythmic difficulties.
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Okay, you're looking at metronomes, but what are they good for?
The Beat Goes On
La De Da De De
La De Da De Da
The first thing is, they can tell you how fast a piece is to be played.  You'll see at the top of many pieces of sheet music a notation like the one above. This one tells you that there should be 128 quarter notes with a value of one beat each (you'll get that part from the time signature) per minute.  And as I listen to "The Beat Goes On" on my computer, that's about how fast it is going.  A little faster than two beats per second.  So I could set my metronome to that speed.

But let's start earlier than that. When I have mastered the basics of a piece of music, I can start playing it with a metronome set much slower than the piece is written. In the case of The Beat Goes On, I might start at 60 beats per minute.  If I can't play it easily there, I set the metronome even slower.  (Violin Virtuosa Rachel Barton-Pine told me that when she talks with students she tells them to start out "SLOW," and the first time they play the piece she immediately says, "No, I said 'Slow.' That was Medium.  Start slower."  Eventually I find a place where I am completely comfortable and also completely successful playing the piece through. 

When I am confident that I can play the piece through correctly, I turn up the speed A LITTLE BIT, maybe 5 or 10 beats a minute, and I work through it again.  When I am completely successful at the new speed, I can move the setting again.  And I do that over and over again until I am playing comfortably at the right speed.  This is not generally a one-day process. It can last days, or weeks, or with some of the quicker things I've played it has lasted a lifetime! 

Until I have finally reached the point where I am satisfied with my ability to play the piece at speed, I don't really concentrate on the musicality of changing speeds within a piece. I'll practice ritards and changes of pace somewhat just to get the feel of what the piece will be like when I have it under control, but  the concept of rubato remains just a thought until my hands can handle the technical aspect of the music.
The shirt talks about the song which has become the standard for a fiddler, Orange Blossom Special. The question is asked, "Can You Keep Up?"  Orange Blossom Special is quick and somewhat technically demanding, but with a plan of metronome use and wise increases of speed, your answer can be..."Want to Race?"

What to look for in a metronome?
It Must Be Loud Enough to be Heard Easily.  (When I'm Playing, Not Just Sitting Still in a Quiet Room)
It Must Be Tough Enough to Handle the Abuse I Give It.  (Will You Be Carrying it in Your Case or Leaving on the Music Shelf at Home?)
It Must Have the Features I Want. (Built In Tuner?)

If you don't have a metronome, ask your teacher about it. Ask if there is one he or she recommends. A good metronome is one of the best investments that you can make in your music.
Orange Blossom Special Shirt