Picks -or plectra- are basically small, flat pieces of plastic employed to strum or pluck the strings. Guitarists playing rock, jazz, and derived genres employ picks more often than those coming from the classical tradition, who play with their fingers. This is by no means a rule, however, as modern guitar technique evolves rapidly, and nowadays a number of hybrid techniques are used by some players.
Picks are usually made of plastic -though other materials may be found, especially in non-guitar picks- and cut in a triangle shape with rounded corners. Pick manufacturers usually print their logo and the thickness of the plastic, which is an important factor in terms of control and precision. Heavy picks usually yield better results, with a more defined attack, and a greater capacity for speed playing (they are less prone to bending). They require more practice to be mastered, however, so its usually recommended that beginners start with thin picks (which as a matter of fact are always good for strumming, because of the particular clicking they emit against the strings).
The capo, short for capotasto - Italian for the 'nut' of the instrument - is a device that is placed on the guitar's fretboard in order to reduce the total length of the strings, creating a new 'artificial' nut. In other words, it acts as if the player was pressing the six strings in line, pretty much as it happens with the fretting hand index finger in a barred chord, basically raising the pitch of the open strings.
As the capo leaves all the left hand fingers free to play, musicians use it to easily transpose any piece or chord sequence without having to memorize new fingerings. Because of this, a capo is an invaluable tool for any accompanist or singing guitarist, as it will allow to easily change the key of a song in order to better suit the singing voice.
There are many designs, sizes and shapes of capos (as they are also used on bass guitars, banjo, mandolins, etc.) Strap-on capos, spring-operated capos... the basic mechanism is always the same: a rubber bar covers the instrument's fretboard, holding the strings down at a given point. Some mechanisms may clamp the strings more evenly, be more reliable than others, or more resistant to time. Usually strap-on capos are the cheapest and they tend to loose their grip with time.
The capo is widely used in several folkloric musical expressions throughout the western world, but rarely seen in jazz and classical music.
A metronome is a tool that produces regular clicks, and it is used by musicians to keep a steady beat. It is an invaluable tool to develop a internal sense of tempo, and exercises featuring the metronome are recommended for students of any instrument since the beginning of their career.
The metronome is also used as a reference between musicians, as many pieces bear 'metronome marks' to objectively describe the tempos the composer intended. Some metronomes include a visual reference, like a swinging pendulum. Nowadays they are usually small electronic devices.
The tuner is an electronic tool used to detect the pitch of a given note, and indicate how far it is from the desired pitch. Simple tuners use a set of led lights to indicate the musician to tune higher or lower, but there are complex devices that give precise details about the frequencies involved, and offer several features such as temperament options, or different tunings. They receive the signal either via a microphone or a line in jack.
Tuners come in a variety of sizes and shapes. They can be small enough to be fitted into a pocket, into a stompbox, or be the size of a rack unit, depending on the construction and complexity of the particular unit. They may work by means of a microphone, a line in jack, picking up vibrations, etc.
They are an useful tool for the beginner guitarist that still has to master the skill of ear-tuning. Tuners can also be invaluable accessories for live settings, when the circumstances don't allow tuning by ear (tuners with the option of connecting a line to them allow for silent tuning).
A strap is a strip of material that allows holding the guitar using the shoulders. Most professional straps are wide, adjustable in leght, and padded, in order to better distribute the weight of the instrument and minimize strain. Usually guitars come with strap pins, where the strap can be connected. Most classical guitars will not feature strap pins at all, in which case the player must either modify it or use a neck strap, which supports the guitar by hooking into the sound hole. Strap locks are sometimes used by guitarists as a safer alternative to strap pins.
A stand allows sheetmusic to be read from an appropriate distance, whether whether it be during performance or study sessions. For those musicians making heavy use of printed sheetmusic, its almost a must to own a proper stand, as it allows a correct performing posture. Reading from a table or desk, or leaning the sheetmusic against something, may cause neck strain or make it hard to appropriately turn the pages. There are a good number of stand designs, prepared to hold different sized scores, and with different transportation capabilities. Guitarists often employ light folding stands, which are convenient for touring. however, usually these can't support heavy books.
A guitar stand is designed to support a guitar: they come in folding and non folding versions, and may include an additional arm for supporting the guitar neck. Besides their immediate function of supporting unused guitars onstage, they can also play a part in developing a consistent study routine, as they allow to have the guitar always accessible and outside its case.
A support is a device used to position an acoustic guitar for playing in the classical posture, but avoiding the use of a stool.
Armrests are devices placed on the top of the guitar to avoid damage from perspiration, as well as direct contact with the top of the guitar (thus letting it project sound better).
The images used in this article are Copyright Free / under Creative Commons Licenses.
Capo: photo by Andrzej Barabasz CC BY-SA 3.0, source & license details: http://commons.wikimedia.org/wiki/File:Capo_(close-up).jpg | Tuner: photo by Cristoph Lange CC BY 2.0, source & license details: http://commons.wikimedia.org/wiki/File:Chromatic-tuner.jpg | Metronome: photo by Paco Vila CC BY 2.0, source & license details: http://commons.wikimedia.org/wiki/File:Wittner_metronome.jpg | Strap: photo by movethetiger CC BY-SA 3.0, source & license details: http://en.wikipedia.org/wiki/File:Couch_Red_80s_Mercedes_Guitar_Strap.jpg | Music stand: photo by Mezzofortist CC BY-SA 3.0, source & license details: http://commons.wikimedia.org/wiki/File:Music_stand_metal.jpg | Guitar stand: CC BY-SA 2.0, source & license details: http://commons.wikimedia.org/wiki/File:Guitar_Epiphone_01.jpg