Guitar

The modern guitar (which probably dates from around the time of the rennaisance) is predated by several other instruments that have the same structural components, such as a set of strings (typically tunable by means of tuning screws, known as machine heads), a neck (either fretted or not) and a soundbox (typically at one end), although some zithers do not have a neck. Some more ancient predecessors are the ancient Greek kythara, lyre, and zither.

Guitars come in different sizes, with the small 4-string ukelele on the smaller side, and the Mexican bass guitar quite large. The baroque guitar had 5 strings, while most modern guitars have 6 strings, with the 12-string guitar basically a 6-string guitar with dual strings per set.

Greek kythara

Source:
http://en.wikipedia.org/wiki/Kithara

Sound-video clip: http://www.youtube.com/watch?v=QYQmJM3r3Bs

Guitar from 800s CE

http://en.wikipedia.org/wiki/guitar.htm

Baroque guitar (1600 - 1800)

Guitar in shape of ancient Greek lyre

English guitar 1750-1800

Source:
http://www.miayf.org/string/plucked-string-instruments/guitars/
http://www.miayf.org/string/plucked-string-instruments/guitars/english-guitar/
http://www.miayf.org/string/plucked-string-instruments/guitars/later-guitars/

Modern nylon string guitar

Source:
http://en.wikipedia.org/wiki/File:Guitar_1.jpg

Ukelele

Source:
http://en.wikipedia.org/wiki/Ukulele
http://en.wikipedia.org/wiki/File:Ukulele1_HiRes.jpg

Mexican bass guitar

Source:
http://en.wikipedia.org/wiki/Guitarr%C3%B3n_mexicano
http://en.wikipedia.org/wiki/File:AcousticBassGtr.jpg

Soundclip

http://en.wikipedia.org/wiki/Acoustic_guitar