Strings are at the heart of how a violin works. A violin has four strings, which are usually tuned to the sounding pitches of G, D, A, and E going from lowest string to highest. The lower strings are thicker than the higher strings. The reason for this is physics. If you take two strings, one thick and one thin, and apply the same tension to both, they will vibrate at different frequencies when struck or bowed. The thicker string will vibrate more slowly, and will sound lower than the lighter string. Length also affects the sounding pitch of a string, because shorter strings will vibrate more rapidly than longer strings.
A violinist uses a bow to draw sound from the strings. The bow is held more or less perpendicular to the strings and is drawn back and forth. As the violinist moves the bow across the string, it sets the string in motion, producing vibration. If the violinist continues to bow the string, his or her energy of motion is adding to the harmonics produced by the vibrating string, creating a rich sound. A violinist can alter the tone of a note by bowing at different places along the length of the string. Bowing closer to the bridge creates a brighter tone, while bowing closer to the fingerboard creates a darker, earthier tone. A violinist can also use a technique called pizzicato, or plucking the violin strings, to produce a bright, short lived note.
Violinists can alter the sounding pitch of their strings in two ways. They can use the tuning pegs on the pegbox to tighten or loosen the tension of the strings, or they can press down on a string anywhere on the fingerboard. When a violinist presses a string to the fingerboard, he or she is shortening its effective length, which causes the string to vibrate more rapidly. Violinists can also rapidly change the sounding pitch of a string by pressing a string against the fingerboard and subtly pushing and pulling the string, which makes the sounding pitch oscillate above and below the central pitch. This technique is called vibrato.
The bridge of a violin is a tall piece of wood which supports the strings and transmits their vibration to the body of the violin. A violin bridge has an arched top, which allows the violinist to play single string passages as well as double stops. The bridge is connected to the body by two feet, which transmit the vibration of the strings to the front plate of the violin body.
The f-holes are named for their distinctive shape. F-holes help make the front plate of the body more responsive to the vibrations of the strings because they create a small area of very flexible material directly underneath the bridge. The f-holes also allow air inside the violin to act upon the air outside the violin, which lets the body cavity act as an amplifier for sound.
A violin body is constructed to resonate with the frequencies produced by the strings. The interior of the violin acts as an amplifier, doubling and redoubling the vibration of the top plate, and increasing the volume of the sound.