"Strings were first made of sheep gut (commonly known as catgut), stretched, dried and twisted. Modern strings may be gut, solid steel, stranded steel, or various synthetic materials, wound with various metals. Most E strings are unwound, either plain or gold-plated steel.
Violinists often carry replacement strings with their instruments to have one available in case a string breaks. Strings have a limited lifetime; apart from obvious things, such as the winding of a string coming undone from wear, a player will generally change a string when it no longer plays "true," with a negative effect on intonation, or when it loses the desired tone. The longevity of a string depends on how much and how intensely one plays. The E string, being the thinnest, tends to break or lose the desired tone more quickly than the others."
"Modern strings are most commonly either a stranded synthetic core wound with various metals, or a steel core, which may be solid or stranded, often wound with various other metals. With low-density cores such as gut or synthetic fiber, the winding allows a string to be thin enough to play, while sounding the desired pitch at an appropriate tension. The winding of steel strings affects their flexibility and surface properties, as well as mass. Strings may be wound with several layers, in part to control the damping of vibrations, and influence the "warmth" or "brightness" of the string by manipulating the strength of its overtones.
The core may be synthetic filaments, solid metal, or braided or twisted steel filaments. The uppermost E string is usually solid steel, either plain or wound with aluminium in an effort to prevent "whistling." Gold plating delays corrosion of the steel and may also reduce whistling. Stainless steel gives a slightly different tone. Synthetic-core strings, the most popular of which is Perlon™ (a trade name for stranded nylon) combine some of the tonal qualities of gut strings with greater longevity and tuning stability. They are also much less sensitive to changes in humidity than gut strings, and less sensitive to changes in temperature than all-metal strings. Solid-core metal strings are stiff when newly replaced, and tend to go out of tune quickly.
While some gut strings still use a knot to secure the tail end in the slot of the tailpiece, most modern strings use a "ball", a small bead often made of bronze, for that purpose. A frequent exception is the E string, which may be had with either a ball or loop end, since the smallest E-string fine tuners hold the tail of the string on a single small hook.
The price of different string types varies dramatically; gut and gut-core strings are typically the most expensive, followed by leading synthetic core brands, and student steel strings at the lowest price range. Natural gut strings (without the metal windings) are quite inexpensive, especially for the e and a strings. The longevity of strings (all types) is highly variable and is influenced by style of play, chemistry of perspiration and its interaction with the string material, presence of fingernails, frequency of play etc. Some players have trouble with certain brands of strings, or one particular string (say an "a" string of brand xyz) but not with others.
The character of the sound produced by the strings can be adjusted quite markedly through the selection of different types of strings. The most noticeable divisions of sound quality for violins is steel, artificial gut ("perlon" core etc), wound gut, and plain gut. The wound gut tend to have a mellow sound, as do many of the artificial strings, though other artificial core strings are specifically designed to be "bright". Steel and plain gut are both rather bright (full of overtones) but in distinctly different ways: you can tell the difference and yet each are more lively or bright typically than the wound soft-core and wound-gut strings. Certain styles of music have come to be played with certain types of strings, yet there is no hard and fast rule in this respect as each musician is looking for his or her sound. (Country fiddling is often on steel or all-metal; orchestral is often wound (gut or artificial) with a steel e; soloists are often like the orchestra; baroque or early music may be played rather more than romantic pieces, on plain gut.)"
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http://en.wikipedia.org/wiki/Violin_construction_and_mechanics#Strings