5. Historical stuff
If we include its ancient predecessors the guitar is pretty old indeed (its name comes from the ancient Greek kithara). The standard 6-string instrument we are familiar with nowadays is a comparatively new invention and doesnt have the long tradition of other band or orchestral instruments.
The 6-stringed form nowadays generally known as classical guitar (with nylon, formerly gut strings), can be traced back to Spanish luthier Antonio Torres in the 19th century. At around this time a German named Christian Friedrich Martin emigrated to the United States and developed the 6-stringed guitar equipped with steel strings.
A precondition for the early design was a body that could withstand a higher string tension. Martin also influenced Orville Gibson who applied violin design (arched top, angled headstock) to the guitar. He founded a tradition which finally culminated in the violin-like Les Paul and became very successful with his Gibson-Mandolin-Guitar company in the process.
During the first decades of the 20th century the guitar was mainly used as a rhythm instrument in orchestras. The guitarists of this period had a hard time standing out against much louder instruments. It is therefore no surprise that playing styles and body size were mainly connected to issues of loudness. Also the concept of the National/Dobro guitar served the improvement of audibility.
Finding that singers were often drowned out by the noise of a full orchestra, it became common practice to position the singer in front of a tube microphone (a mix was finally achieved by varying the distance between singer, orchestra and microphone). It was just a matter of time before someone came up with the idea of placing a microphone in front of the guitar.
The first electrically amplified guitar sounds were produced with acoustic guitars the corresponding problems on louder stages (cross-talk, feedback) are well-known. The remedy was the invention of the electromagnetic pickup, which, of course, only worked with steel strings. Later on the idea of getting rid of the hollow body altogether became apparent. The first experiments with purely electric guitar (still affectionately known as the plank today) appear somewhat basic consisting of a simple wooden plank with a roughly mounted neck and pickups.
A Texan by the name of Beauchamp developed the first official electric guitar pickup by dismantling the pickup of an early record player and exposing the strings instead of the needle to its magnetic field. He was also probably the first to build a guitar without a hollow body. Playing the guitar with a pickup, however, was popularized by a young black guitarist named Charlie Christian whose Gibson ES-150 actually had a hollow body but the additional pickup allowed the virtuoso to play solo parts that would otherwise have been drowned out by the sound of the orchestra.
Other famous examples of early guitars are the frying pan and the broom stick developed by a Swiss emigré named Richenbacher (the Yanks couldnt spell his name properly, so the company was named Rickenbacker the name it still bears to this day).
And so we arrive at the renowned names that rule the guitar world nowadays:
Leo Fender started from scratch repairing radios, record players, guitars and amps. He didnt think much of complex guitar-building concepts because they were incredibly expensive and time-consuming. Together with his co-worker George Fullerton he founded the Fender company. He started by manufacturing Hawaiian and steel guitars using the plank shape still in use today. This became the first really popular electric-only guitar to be mass produced and included a massive flat body with a cutaway to improve the reachability of the higher fret positions, 2 pickups for more sound options and a bolted neck with 4 screws. And so the Telecaster was born! At this time (the 1950s) the new Fender guitar was also called the Esquire, later the Broadcaster and less frequently the NoCaster. It was, however, the growing popularity of television that secured the name which we all recognize today. A few years later the Stratocaster was produced. It was, and remains as a fundamentally unmodified design, probably the most successful electric guitar of all time. Its extreme versatility and unsurpassed design (some of them are even exhibited in museums) were the pivotal factors in this.
At Gibson it took a little longer to take up an opportunity with the planks. After all, unlike Leo, they had a luthiers tradition to preserve. It was a very talented and open-minded guitarist named Lester Pollfuß, better known as Les Paul, who pushed the companys innovative possibilities. He wanted a solid instrument and contributed a few ideas of his own before cashing in on a neat amount of royalties with that famous name of his.
Another important step was the development of the first and ever popular humbucker by Seth Lover in 1957. With the PAF (named after the patent applied for sign underneath) it was possible to eliminate the noise of the single coils for the first time. Interestingly, it was exactly this softer and easily distorting humbucker-sound which nearly finished it for Les Paul in 1960. Clean, brilliant guitar sounds were in demand and so Les Paul production was stopped without further ado. The more modern-looking SG had also been labeled Les Paul until Mr. Pollfuß decided to put an end to it.
Not until blues guitarists like Mike Bloomfield and above all Eric Clapton discovered the Les Paul (the nowadays inestimable sunburst-models with PAFs from the late 50s were dirt-cheap at that time) as the perfect vehicle for their rough modern blues sound in the 60s, did things start looking up again with no end in sight. Every time the Les Paul seems about to be buried in oblivion a new guitar hero comes along, causing a new boom; most recent examples being Slash and Gary Moore in the early 90s.
In 1958 there was another innovation: The still popular semi-acoustic, called ES (Electro Spanish) at Gibson. The semi-acoustic models by Gretsch which number among the classics should not go unmentioned either.
Of course, many other brands and types followed, trying to establish new concepts, but basically almost every guitar type that is popular today traces its roots back to the classics developed in the 50s.