Have you ever had a guitar string that just buzzes on one fret? It can be the most annoying thing in the world. Chances are, you have one fret that is higher than the other frets around it. Your string hits the higher fret and buzzes. You will need a fret rocker to detect which fret is the culprit. A fret rocker is a guitar repair tool used to set up the guitar. A fret rocker is a trapezoid shaped piece of metal with each edge precision ground until it is perfectly straight. The fret rocker is used to measure how level each fret is in comparison with its neighboring frets.
Each edge of the fret rocker is a slightly different length corresponding to the different spacing of the guitar frets. Take your fret rocker and place it up against the frets of your guitar. The fret rocker should only cover three frets at a time. If it is covering more than three frets, simply turn it to a different edge. Once it covers three frets, you will be able to rock the fret rocker back and forth to see if the middle fret is too high. If the rocker doesn't rock, those frets are good. Start at the low frets and work your way up the fretboard until you find that high fret. Once you hit the fret, the fret rocker will visibly rock back and forth one the frets like a teeter tauter. Now you can mark that fret for leveling.
Until the fret rocker was invented, luthiers and guitar repairmen had to have four or five small straight edge rulers to individually check each fret's height. Obviously, this was a pain because you had to keep track of all of these extra tools and you had to have them out on your guitar repair bench when you were doing the repair. The fret rocker combines these four or five straight edges into one luthier tool. It is convenient and extremely useful. I recommend adding one to your guitar repair bench. If you would like more information about how to use a fret rocker and how to level your guitar frets, please see my how to dress guitar frets article.