Adding a minor seventh to a major triad creates a dominant seventh (denoted V7). In music theory, the "dominant seventh" described here is called a major-minor seventh, emphasizing the chord's construction rather than its usual function. Dominant sevenths are often the dominant chords in three-chord progressions, in which they increase the tension with the tonic "already inherent in the dominant triad".
The pickguard, also known as the scratchplate, is usually a piece of laminated plastic or other material that protects the finish of the top of the guitar from damage due to the use of a plectrum ("pick") or fingernails. Electric guitars sometimes mount pickups and electronics on the pickguard. It is a common feature on steel-string acoustic guitars. Some performance styles that use the guitar as a percussion instrument (tapping the top or sides between notes, etc.), such as flamenco, require that a scratchplate or pickguard be fitted to nylon-string instruments.
Guitars have been played since the Renaissance era, after descending from the ancient Greek instrument, the kithara. Or, at least, that's how the theory goes: the only real proof for this is the similarities between the Greek word and the Spanish word, quitarra. Early guitars often had four strings, evolving into the six-string version we know today in the late 1600s. Anton Stradivari, the famous violin-maker, also had a hand in making guitars. There's now only one Stradivarius guitar left in existence.