The Spanish Guitar
In it's earliest form the guitar had three double courses (pairs) of strings plus one single string (the highest). The guitar probably originated in Spain, where by the 16th century it was the counterpart among the middle and lower classes of the aristocracy's vihuela, an instrument of similar shape and ancestry with six double courses.The guitar became popular in other European countries in the 16th and 17th centuries, and by the late 17th century a fifth course of strings had been added below the other four.
In the mid-18th century the guitar attained its modern form, when the double courses were made single and a sixth string was added above the lower five. Guitar makers in the 19th century broadened the body, increased the curve of the waist, thinned the belly, and changed the internal bracing. The old wooden tuning pegs were replaced by a modern machine head.