Examine the neck of your guitar.
You’ll notice there are metal strips running across it’s entire surface. These pieces of metal are referred to as “frets” on a guitar. Now, here’s what you’ll need to keep in mind: the word “fret” has two different meanings when used by guitarists. It can be used to describe:
1. The piece of metal itself
2. The space on the neck between one piece of metal and the next
To further explain, the area of the neck between the nut and the first strip of metal is referred to as the “first fret”. The area on the neck between the first and second strip of metal is referred to as the “second fret”. And so on…
The neck of a guitar includes the guitar’s frets, fretboard, tuners, headstock, and truss rod. The wood used to make the fretboard will usually differ from the wood in the rest of the neck. The bending stress on the neck is considerable, particularly when heavier gauge strings are used (see Strings and tuning), and the ability of the neck to resist bending (see Truss rod) is important to the guitar’s ability to hold a constant pitch during tuning or when strings are fretted. The rigidity of the neck with respect to the body of the guitar is one determinant of an instrument’s quality. Conversely, the ability to change the pitch of the note slightly by deliberately bending the neck forcibly with the fretting arm is a technique occasionally used, particularly in the blues genre and those derived from it, such as rock and roll. The shape of the neck’s cross-section can also vary from a gentle curve to a more pronounced “V” shape. (The fretboard is typically gently rounded across its width.)
Marker dots on the face of the fretboard are usually placed at frets 3, 5, 7, 9, 12 (double dot to indicate the octave), 15, 17, 19, 21, 24 (double dot to indicate the second octave). It’s also common that there are marker dots on the side of the neck, near the edge of the fretboard, where the player can easily see which fret he or she is on. Sometimes the dots are replaced with bars, the octave positions having a wider bar. Classical guitars almost never feature position markers, especially on the fretboard’s face, whereas electric guitars usually do.