what are the string instruments, what do they consist of?
String instrument
String instruments, stringed instruments, or chordophones are musical instruments that produce sound from vibrating strings when the performer plays or sounds the strings in some manner. 
Musicians play some string instruments by plucking the strings with their fingers or a plectrum—and others by hitting the strings with a light wooden hammer or by rubbing the strings with a bow. In some keyboard instruments, such as the harpsichord, the musician presses a key that plucks the string.
With bowed instruments, the player pulls a rosined horsehair
chords with melodic embellishments. McCartney, reluctant to repeat what he had done on Yesterday, explicitly expressed that he did not want the strings to sound too cloying. For the most part, the instruments "double up"—that is, they serve as a single string quartet but with two instruments playing each of the four parts. Microphones were placed close to the instruments to produce a more biting and raw sound. Engineer Geoff Emerick was admonished by the string players saying "You're not supposed to do that." Emerick was starting