The horizontal lines represent the frets on the guitar and the vertical lines represent the strings. The numbers listed let you know what fret number it is referring too. If there is an X it means don't play that string and if there is a 0 it means to play that string but do not press down on it anywhere. The black circles show you where to press down.
Modern pickups are tailored to the sound desired. A commonly applied approximation used in selection of a pickup is that less wire (lower electrical impedance) gives brighter sound, more wire gives a "fat" tone. Other options include specialized switching that produces coil-splitting, in/out of phase and other effects. Guitar circuits are either active, needing a battery to power their circuit, or, as in most cases, equipped with a passive circuit.
In major-thirds (M3) tuning, the chromatic scale is arranged on three consecutive strings in four consecutive frets. This four-fret arrangement facilitates the left-hand technique for classical (Spanish) guitar: For each hand position of four frets, the hand is stationary and the fingers move, each finger being responsible for exactly one fret. Consequently, three hand-positions (covering frets 1–4, 5–8, and 9–12) partition the fingerboard of classical guitar, which has exactly 12 frets.[note 1]
Some piezo-equipped guitars use a hexaphonic pickup. "Hex" is a prefix meaning six. A hexaphonic pickup produces a separate output for each string, usually from a discrete piezoelectric or magnetic pickup for each string. This arrangement lets on-board or external electronics process the strings individually for modeling or Musical Instrument Digital Interface (MIDI) conversion. Roland makes hexaphonic pickups for guitar and bass, and a line of guitar modeling and synthesis products. Line 6's hexaphonic-equipped Variax guitars use on-board electronics to model the sound after various vintage instruments, and vary pitch on individual strings.
Guitars have been played since the Renaissance era, after descending from the ancient Greek instrument, the kithara. Or, at least, that's how the theory goes: the only real proof for this is the similarities between the Greek word and the Spanish word, quitarra. Early guitars often had four strings, evolving into the six-string version we know today in the late 1600s. Anton Stradivari, the famous violin-maker, also had a hand in making guitars. There's now only one Stradivarius guitar left in existence.