A string is the element of musical instrument, which creates sound in the result of vibration. A string is a narrow strip of flexible material which is fixed in a musical instrument in a way it can freely vibrate, but that vibration remains controllable.
- 1 String structure
- 1.1 Winding
- 1.1.1 Round-wound
- 1.1.2 Half-rounds
- 1.1.3 Flat-wounds
- 1.2 Core
- 1.2.1 Round Core
- 1.2.2 Hex Core
- 2 Gauge
- 3 String materials
- 3.1 Core materials
- 3.1.1 Steel
- 3.1.2 Nylon
- 3.2 Winding materials
- 3.2.1 Bronze
- 3.2.2 Phosphor Bronze
- 3.2.3 Pure Nickel
- 3.2.4 Nickel Plated (Nickel Wound)
- 3.2.5 Stainless Steel
- 4 Coating
- 5 See also
Strings may be plain or wound. Plain strings consist of a single material. Wound strings have a core of one material, with an overwinding of other materials.
On 6-string electric guitars, the top three strings are wound (on 6-string acoustic guitars - four). Every manufacturer uses different winding processes (core to wind ratio, winding tension), but there are 3 main winding types.
This type of winding is used in the majority of guitar strings. Round wire wrapped is a spiral around the string core, making the string easier to grip. The drawback is that there is more string noise when changing notes and chords.
Round wound strings are simple to manufacture and less expensive than other types.
Half-rounds are polished for a smoother winding. There is less noise when changing notes, and still some grip. Polished strings produce less treble and more warm sound.
Flat-wounds almost resemble plain, unwound strings. There's almost no noise when changing notes. They create softer, mellow tones.
There are two main types of string cores: round and hex.
For round core strings, the core is simply a round wire. Round core wires are considered easier to bend and produce a warmer sound.
Hex core is a hexagon-shaped (six-sided) wire. Hex core wires have a faster response, because of the way they are locked to the winding. They are also more consistent in terms of quality. Most strings use hex cores.
Gauge is the diameter of the string. Usually, string gauge is measured in thousandths of an inch.
String gauge affects the playability and output of the guitar. The heavier gauge the strings are the tighter they must be, therefore making it harder to fret and bend notes. The benefit of heavy gauge strings is that they produce louder tones than light gauge strings.
Another thing to consider is the scale length of the guitar. The longer the scale the tighter the strings have to be. String gauges commonly vary from .008 on the 1st string, to .056 on the 6th string.
When matching string gauges, you often hear descriptions such as:
- Extra Light – (.009/.011/.016/.024/.032/.042)
- Light – (.010/.013/.017/.026/.036/.046)
- Medium – (.011/.015/.018/.026/.036/.050)
To make things easier, guitarists typically refer to a whole set of strings by the gauge of the high E string. So a set of medium strings would simply be an “11“.
But there are no established definitions for any of these terms.
- Light strings on an electric guitar will have smaller gauges than light strings on an acoustic guitar.
- And light strings for either can vary greatly from one manufacturer to another.
For classical strings, the situation is a bit different. While the specific gauges are still shown, they aren’t nearly as important as the string "tension". There are 3 standard options to choose from: low, medium, and high tension.
Heavier gauges are generally better for:
- heavy strumming as they have more durability, more sustain, and less breakage.
- slide playing and drop tunings as they provide a tighter string tension.
- low-action guitars – because they have tighter vibrations, and are therefore more resistant to fret buzz.
- unamplified acoustic playing – because they're louder.
- Jazz – because that style of music doesn't use much note bending. 
Lighter gauges are generally better for:
- beginner playing as they are easier to fret.
- blues and soloing as they allow easier bending.
- vintage guitars – because of lesser stress on the guitar neck.
- small-body guitars as these strings have higher resonance.
- fingerpicking, because such strings are more responding to accurate fingerwork. 
Many companies also propose a "hybrid gauge" light-medium strings, which have lighter gauges on G, B, E and heavier gauges on E, A, D. These strings are designated for players who use a good mix of picking and strumming.
Different combinations of used materials affect the way strings will sound. Music genres highly affect the choice of strings made of different materials.
Steel or metal strings is the main type of strings for the electric guitar and bass. They have a bright tone as compared to nylon string guitars.
Nylon strings are used in classical guitars and have a more mellow and muffled tone.
Bronze strings are the brightest sounding strings (for acoustics). They lose their tone quickly, though. These may be referred to as 80/20 Bronze (80 percent copper, 20 percent zinc).
Phosphor bronze strings have a higher amount of copper (92/8, usually) than bronze strings. Such strings produce a warmer tone.
Pure nickel strings have a warm, vintage tone. The magnetic field is not as strong as nickel plated strings, so volume levels will be slightly lower on electric guitars.
Nickel Plated (Nickel Wound)
These strings will create higher volume levels than pure nickel can. Nickel plate strings also have greater sustain and brighter tone.
Strings made of stainless steel have a more aggressive sound than nickel or nickel plated. The downside is that they will wear frets down quicker unless your frets are also stainless.
Main article: Coating
Coated strings have a layer of protectant around them that keep dirt and corrosives off the strings, making them last longer. 
Strings are usually protected by a polymer coating. The polymer is often a Teflon PFT (Polytetrafluoroethylene). Typically the wound strings are coated. The polymer coating was originally developed to prevent string corrosion. Specifically, highly corrosive strings such as bronze acoustic strings are almost impossible to keep fresh sounding without a protective polymer layer. By keeping the alloy oxygen free, the strings can sound fresh out of the box for months. Whereas uncoated bronze strings can sound dead after a much shorter duration of time. 
- ↑ 1 2 E-Home Recording Studio "The Definitive Guide to Guitar Strings for Acoustic/Electric Guitar"
- ↑ Ultimate Guitar "Beginner's Guide to Guitar Strings"
- ↑ ProfessorString.com "The Five Things You Must Know About Coated Guitar Strings"