Welcome to Muppet Wiki!
The String Quartet is a routine performed by the Muppets on a number of variety shows. It was first performed on The Today Show on March 30, 1962. In this performance, Kermit tries to conduct a string quartet, consisting of Harry the Hipster on drums and Theodore on piano, but the violinist is missing, and both Harry and Theodore perform different songs at the same time. Eventually, a woman named Mildred runs from backstage to bring in violets, which she thought was needed as opposed to violins.
The sketch was changed a bit when it was performed on The Ed Sullivan Show on January 16, 1971. This time the string quartet members include Twill (the leader), Grump, Harrison, and the unseen Beegleman, who can't make it and is therefore replaced by Mahna Mahna (named "Harry" in the script), who plays the drums instead of the violin, and makes many wisecracks and puns during the performance, but in the end the entire quartet gets along together.
Performers in the performance on The Ed Sullvian Show included Jim Henson as Mahna Mahna, Jerry Nelson as Twill, Frank Oz as Grrump, and Richard Hunt as Harrison.