Acoustic guitars, with hollow bodies usually but not always made of wood, are available in many different models. Acoustic guitars come in two main categories: the classical and the folk guitar. Guitar-makers and guitarists can customize and adapt instruments in many different ways, but a few general differences separate them.
One important difference between folk and classical guitars is the material used for the strings. Classical guitars are strung with nylon. In folk guitars, the strings are steel. The use of steel strings, in addition to giving a sharper, more strident tone, greatly increases the volume of the guitar's sound. This was desirable before amplification, especially for folk musicians playing with bands in informal dance hall settings rather than in hushed concert halls. The use of steel strings became possible in the 19th century with the development of the "x-brace," a structure under the soundboard which reinforces the guitar and helps it withstand the extra pressure exerted by steel rather than nylon strings. Christian Fredrich Martin, founder of Martin guitars, was responsible for the innovation. In addition, classical guitars usually have a longer string length from head to bridge.
The "action" is the name for the distance between the fingerboard or fretboard and the strings; in other words, how high the strings are. Classical guitars have a higher action than folk guitars thought to improve tone and increase the possibility of sustaining notes. A higher action requires more pressure on strings when fingering notes, but in classical guitars, this is compensated for by the much lower tension of nylon strings. High tension steel strings on folk guitars typically have a lower action for ease of playing. Individual guitarists can vary the action on any guitar to suit their personal preference.
Classical guitars also have wider necks than folk guitars. This allows the strings to be spaced more widely apart, an important factor for classical guitarists who need to play individual notes with their fingers, sometimes at great speed. The wider spacing of the strings makes it easier for the fingers of the hand (in a right-handed guitarist) to be inserted between the strings to pluck them. Fingering chords with the left hand is easier, especially for players with smaller hands, on narrow-necked folk guitars. Because the music often calls for chords rather than individual notes and picks are more commonly used, widely spaced strings are less necessary.
Nylon strings and longer string length are responsible for the distinctive sound of classical guitars, described as round, full, gentle, mellow, with deeper bass tones. In contrast, steel-stringed folk guitars have a bright, assertive, percussive timbre with strident treble. The contrast in tone is appropriate to the styles of music for which the guitars are intended.
Classical guitars are often recommended for beginners, mainly because low-tension nylon strings are easier to finger than steel strings. Since the price of both styles of guitar varies from inexpensive to very costly for the top models, a player's choice is usually based on type of music. Most find that the mellow tone of classical guitars does not lend itself to folk, pop or rock styles, and will graduate to steel-stringed folk guitars or steel-stringed electric guitars if that is the musical road they choose to follow.