What is Kramer's first name, revealed in The Switch?

In “The Switch,” Kramer's first name is revealed to be Cosmo.

For the first five seasons, the character is almost always called simply Kramer, and the show treats his given name as a running mystery. The secret is finally exposed when Kramer reconciles with his estranged mother, Babs, who addresses him as “Cosmo” in front of George and the others.

The revelation was deliberately protected before the episode aired. NBC promoted the mystery, and production scripts reportedly replaced the name with Xs to prevent it from leaking. Afterward, the characters still generally called him Kramer, so Cosmo became an established but comparatively infrequent part of his identity.

The name was suggested by Larry David, who borrowed it from a boy living in the same apartment building as David and the real-life Kenny Kramer, the neighbor who inspired the character. An unused episode concept would have given Kramer the first name Conrad, but that version was never filmed.