The numbers 0 - 12 on the chart below represent the fret numbers.
Also called the fingerboard, the fretboard is a piece of wood embedded with metal frets that comprises the top of the neck. It is flat on classical guitars and slightly curved crosswise on acoustic and electric guitars. The curvature of the fretboard is measured by the fretboard radius, which is the radius of a hypothetical circle of which the fretboard's surface constitutes a segment. The smaller the fretboard radius, the more noticeably curved the fretboard is. Most modern guitars feature a 12" neck radius, while older guitars from the '60's and '70's usually feature a 6" - 8" neck radius. Pinching a string against the fretboard effectively shortens the vibrating length of the string, producing a higher pitch. Fretboards are most commonly made of rosewood, ebony, maple, and sometimes manufactured or composite materials such as HPL or resin.
Frets
Frets are metal strips (usually nickel alloy or stainless steel) embedded along the fretboard which are placed in points along the length of string that divide it mathematically. When strings are pressed down behind them, frets shorten the strings' vibrating lengths to produce different pitches- each one is spaced a half-step apart on the 12 tone scale. For more on fret spacing, see the Strings and Tuning section below. Frets are usually the first permanent part to wear out on a heavily played electric guitar. They can be re-shaped to a certain extent and can be replaced as needed. Frets also indicate fractions of the length of a string (the string midpoint is at the 12th fret; one-third the length of the string reaches from the nut to the 7th fret, the 7th fret to the 19th, and the 19th to the saddle; one-quarter reaches from nut to fifth to twelfth to twenty-fourth to saddle). This feature is important in playing harmonics. Frets are available in several different gauges, depending on the type of guitar and the player's style.
Most guitars have frets on the fingerboard to fix the positions of notes and scales, which gives them equal temperament. Consequently, the ratio of the spacing of two consecutive frets is the twelfth root of two sqrt[12]{2}, whose numeric value is about 1.059463. The twelfth fret divides the scale length in two exact halves and the 24th fret (if present) divides the scale length in half yet again. Every twelve frets represents one octave. In practice, luthiers determine fret positions using the constant 17.817152, which is derived from the twelfth root of two. The scale length divided by this value yields the distance from the nut to the first fret. That distance is subtracted from the scale length and the result is divided in two sections by the constant to yield the distance from the first fret to the second fret. Positions for the remainder of the frets are calculated in like manner.
There are several styles of fret, which allow different sounds and techniques to be exploited by the player. Among these are "jumbo" frets, which have much thicker wires, allowing for a lighter touch and a slight vibrato technique simply from pushing the string down harder and softer, "scalloped" fretboards, where the wood of the fretboard itself is "scooped out", becoming deeper away from the headstock, which allows a dramatic vibrato effect and other unusual techniques, and fine frets, much flatter, which allow a very low string-action for extremely fast playing, but require other conditions (such as curvature of the neck) to be kept in perfect order to prevent buzz.