Fret markers are a visual reference point to find frets quickly. They are traditionally placed on the 3rd, 5th, 7th, 9th, and 12th frets. The twelfth fret usually has a double-dot marker. The number of frets varies from one guitar to another, but all should have at least twelve. Why "at least twelve"? With twelve frets you can produce one full octave on each string (an octave contains 12 distinct notes).