"As a musical instrument, Spanish guitar is also called a classical guitar. However, musically it is a style unto itsself. Spanish guitar had its roots in 2 camps, Vienese Classical music and traditional Flamenco music. Early Spanish composers like Gasper Sanz wrote in the Vienese classical style and would not be recognized today as 'Spanish' guitar. Later Spanish guitar composers like Francisco Tarrega were influeced by both camps and that marrige of these 2 styles gives us the 'Spanish' guitar sound we recognize today; beautiful, melancholy, and passionate. 

It’s sister music, Flamenco comes from non-academic folk music traditions. Flamenco sprang from the lower levels of Andalusian society, and thus lacked the prestige of art forms among the middle and higher classes, but many of the songs in flamenco still reflect the spirit of desperation, struggle, hope, and pride of the simple lower class people. Musically it is also very interesting. Disregarding the rules of Western classical music whereas only the major and minor modes have remained, flamenco uses the Phrygian mode or 'flamenco mode' most commonly charactarized by a flatted 2nd scale degree. This mode leads to interesting harmonic findings, with unusual unresolved dissonances. Examples of this are the use of minor 9th chords for the tonic. The aural result is one of tremendous power, passion, and freedom." - Darren Curtis Skanson