Mellotron

The Mellotron is a keyboard instrument that gets its sound by using tape relays. You can get many different sounds from a mellotron, but the only one anyone ever uses is the string ensemble. After being used by the Moody Blues and King Crimson, the instrument became one of prog's defining characteristics. Pretty much every prog band uses one. Seriously, name a prog band that doesn't use a mellotron ever. A lot of not prog but still 70s bands used them too, like Led Zeppelin. The Mellotron was in production from 1963 until 1986, and later went back into production around 2007 with digital models. The Mellotron, along with the Hammond Organ, is one of Prog's defining keyboards.

Only Keith Emerson was stupe enough to never even touch one, perhaps leading him to be pissed on decades later.