The Spanish guitar is built for extreme rigidity. The wood used in the bodies of Spanish guitars is much thicker than that in the classical guitar. The carved arched top(front)and back design of the Spanish guitar add greatly to the demensional stability to the body. The f hole design is required because the Spanish guitar's body is reinforced with a massive strut called a bass bar which runs inline with the neck across the length of the body. The bass bar would impede the free passage of air if the circular hole of the classical guitar was used. The bass bar is usually glued to the sounding board to provide additional rigidity to the instument.
The need for two such different guitars becomes obvious upon use. The classical guitar has a warm mellow sound rich in overtones and undertones and moderate sustain. The classical guitar's weaknesses are that the body may deform over time, or even in the act of being played and therefore may be noticably out of pitch when played in the higher neck area. Also,due to its mellow sound, it is easily overwhelmed when played alongside other instruments. The Spanish guitar has a thinner clearer tone, excellent sustain, much greater accuracy in the upper register and can stand out when played alongside other instruments.