There is no conclusive answer in the debate about which was the first rock and roll record, but there is no argument about rock's importance in the latter half of the 20th century. Prior to the rock era, musical genres within popular music were frequently defined by the instruments they used. Now, contemporary music is built around the four basic instruments that the rock era popularised.
There is no instrument more synonymous with rock music than the electric guitar. Originally developed as a way for the player to be heard alongside louder acoustic instruments, the distinct sounds of the electric guitar and amplifier became a focal point of the rock style. The type of distortion created by guitar and amp, or lack of it, has become one of the sonic elements that now define subgenres within rock music. Despite the importance of the electric guitar, acoustic guitars remain an important rhythmic instrument.
While the guitar seems to define rock music, the history of the electric bass most closely parallels that of the rock era. First mass-produced in the early 1950s by Leo Fender, so dominant were his instruments that any manufacturer's instruments were called "fender basses" to distinguish them from upright acoustic basses. The low level underpinning of rock bass is just as ubiquitous as the guitar, but serves a foundation role as part of the rhythm section with the drums.
Driving the danceable offbeat that defines rock music, the modern drum set completes the essential triumvirate of rock instruments. The basic kit consists of bass and snare drums, two or more tom-toms, hi hat and cymbals. Additional percussion instruments such as cow bell, tambourine, shakers and gongs are often added to allow the drummer to cover a wide range of sounds and style.
The rock trio is often augmented by a second guitar or some form of keyboard. Pianos and organs date to the early days of rock, soon followed by electric pianos like the Rhodes and Wurlitzer. Synthesizers developed in the 1960s became popular quickly and defined an entire genre by the early '80s. With the ability to sound like a wide range of other instruments as well as the ability to create entirely new sounds, keyboards remain popular in rock music.
No family of instruments is off limits to rock music. Strings and brass are often used for filling out arrangements. The saxophone makes frequent visits to the rock and roll stage, and all orchestral, folk and world music instruments have been experimented with in rock's context. While its essence comes down to three or four instruments, the versatility of rock is part of the reason the genre has endured.