The evolution of the modern guitar is a complicated one. Stringed instruments with some of the features of modern guitars have been used for about 4,000 years, as can be attested by archaeological evidence from digs in Egypt, Greece, Mesopotamia, and elsewhere where organized human civilizations have been found.
The guitar didn’t really start to take its modern shape until the beginning of the Renaissance – somewhere around the year 1,000 A.D. The first “guitars” usually had four strings and a fretted neck. But development was slow. The first music written for the “chitarra” dates from around the 16th century.
A five-course instrument with 5 sets of identically tuned strings (10 strings in total) first appeared in Italy in around the 16th century, and gradually replace the 4 string version throughout Europe.
Interestingly these instruments were tuned like the top 5 strings of modern guitars: A-D-G-B-E – which further supports the idea that this is a “natural” configuration based on the fact that our fingering hand has 4 fingers.
A sixth course of strings was added in Italy sometime in the 17th century. and the dual course strings eventually gave way to single strings. This format was gradually adapted throughout Europe.
The final evolutionary step took place in Spain. As one account puts it:
“The modern “classical” guitar took its present form when the Spanish maker Antonio Torres increased the size of the body, altered its proportions, and introduced the revolutionary “fan” top bracing pattern, in around 1850. His design radically improved the volume, tone and projection of the instrument, and very soon became the accepted construction standard. It has remained essentially unchanged, and unchallenged, to this day.
For more information and photographs of ancient guitar ancestors see the excellent article titled “A Brief History of the Guitar” by Paul Guy.