A fret is a raised marker on the neck of a stringed instrument, typically fixed in place and made of hard-wearing material such as metal. They divide the neck into fixed segments, relating to a musical framework. For example, on instruments such as mandolins, each fret represents one semitone relating to the standard western system where one octave is divided into twelve semitones.
Pressing the string against the neck reduces the vibrating length of the string to that between the bridge and the next fret between the fretting finger and the bridge, thus changing the pitch of the note produced.
The advantage of frets is that they make it much easier to achieve an acceptable standard of intonation. However, those who favour fretless stringed instruments will be quick to point out that this means accepting certain compromises. However, for those without the blessing/curse of perfect pitch they provide an easier way to make relatively harmonious music.
Since the intonation of most fretted instruments is equal tempered, the ratio of the widths of two consecutive frets is , whose numeric value is 1.059463. The twelfth fret divides the string in two exact halves.
Slanted frets: Most instruments have frets that are perpendicular to the neck. From the point of view of the anatomy of the human hand however slanted frets would be more comfortable. Even so only one guitar maker, namely Rickenbacker, makes guitars with slanted frets. The frets at an angle of (what angle again?) with the neck. Note that slanted frets are available from Rickenbacker on only a few models and then only as an option. (I don't know if slanted frets are available for left-handed models even when they are available for the corresponding right-handed models.) Rickenbacker's slanted frets models carry an SF after the model number (e.g. Rickenbacker 360/12 SF).
Fretlessness: Many fretted instruments are also available in fretless versions. For example, the bass guitar and acoustic bass guitar come in both varieties. The majority are fretted, like the vast majority of guitars, but a significant number of fretless basses are also played, in part drawing on the influence of the double bass (itself, virtually always fretless).
It is also possible to find semi-fretted instruments, although these are normally one-off, custom adaptations made for players who want to combine elements of both types of sound. One arrangment is for the frets to only extend part of the way along the neck so that the higher notes can be played with the smooth expression available on a fretless instrument. Another approach is the use of frets that exend only partway across the fretboard so that some courses of strings are fretted and others fretless, for example Ryszard Latecki's Latar .
Another variant is called 'scalloping' and involves the wood between some or all of the frets being scooped out. This allows a lighter touch for faster playing and also opens up new options for altering the pitch by bending the strings with the fretting hand. It had some popularity with musicians playing heavy metal music although the ideas can also be seen in more ancient instruments such as the sitar.
- Illustrations and discussion of semi-fretted basses at Talkbass.com