Core Materials
The core affects the sound and bow response of the string.

Gut core strings produce the darkest, richest sound. Since gut is a natural fiber, it will react to changes in temperature and humidity. Response is slower than other string types. Use with fine tuners is not recommended.

Synthetic core strings are meant to sound as similar to gut as possible, but with superior stability and durability. Stronger than gut, synthetic strings are not as susceptible to changes in humidity. Sound is generally brighter than gut, with a faster response. Use with fine tuners is not recommended.

Steel core strings are brighter in tone and more responsive than gut or synthetic strings. They consist of a single strand of steel that is not as susceptible to climate changes and will generally last a long time. May be used with fine tuners.

Rope core strings are brighter than steel. The core consists of many thin strands of steel. Fastest response of all the strings, not as susceptible to climate changes. May be used with fine tuners.

Gauge

Manufacturers use different terminology in describing string gauges.  Soft, light, medium, heavy and thick are a few terms used to describe gauge. 

Soft, Light, Dolce, Weich, Thin - Lower tension, low bow pressure, and a brighter sound.

Heavy, Thick, Stark, Forte - Higher tension, more bow pressure, and a more powerful sound.

Medium, Mittel - The most popular string gauge used because it balances the tension and sound qualities. 

String Materials

Common materials used in string production include Titanium, carbon steel, chromesteel, silver, tungsten, and gold-plating.  Here are some advantages of various string materials.

Titanium - Lightweight and yields a warmer tone.

Carbon Steel - Extremely durable

Chromesteel - Easy response and a lively sound.

Tungsten - Used to increase volume output.

Gold-plating - Primarily used on violin "E" strings for its sweet solo characteristics.

Q&A: E-Strings for Violinists

Charles Avsharian on E-Strings:

Q: What do you look for in an E-string?
A: Tone quality and durability. I happen to prefer a really brilliant sound. I am currently using a Corelli Crystal E which may be too bright for some musicians... they are powerful and sound great on my Lupot.

Q: How often do you change E-strings?
A: I have had some last for many months.

Q: What about thickness and wear on your fingers?
A: One has to decide about the gauge/thickness by trial and error. A thick string will add pressure to the top of the violin - could be good or bad. I use a medium gauge.

Q: What is it that is most important to you about an E-string?
A: CLARITY. Pure ringing and responsiveness all the way to the end of the fingerboard. A PLEASANT SOUND. When played alone, (open string - not a cheap and annoying tone). AS SMOOTH AS POSSIBLE. Not producing a strident, ugly sound. POWER. Especially up high where the brilliance is absolutely needed and expected. LONG-LASTING. Not prone to breaking or becoming false quickly. "WHISTLING" RESISTANCE.

Q: Any other comments to any one considering trying a new E-string?
A: If you can't stand whistling... try the 'wound' E-strings.

Additional Information for Bass Strings

-          ‘Extra Long’  = E string for a bass with an extension

-          Orchestral Tuning = G D A E

-          Solo Tuning = A E B F#

-          5 string bass tuning  G D A E (B or C)

-          6 String Bass tuning  C G D A E (B or C)