The Accordion is a portable box-shaped musical instrument of the hand-held bellows-driven free-reed aerophone family, and is sometimes referred to as a squeezebox.
One is owned by Rolf and first seen in "Key to My Ed." It is likely an antique, as Rolf mentions it has been in his family for generations. Some of its features are organ pipes and a horn on the side. Eddy seems to find it annoying while Ed enjoys it. Its case was one of the things the Eds attempted to open with Ed's Key. Rolf also owns a concertina, a smaller different type of accordion.
An accordion was heard as a backing to the song That's My Horse in "Wish You Were Ed," but the instrument itself was not seen.
His accordion appeared again in "Mission Ed-Possible" as one of the contents of his bag. It was briefly seen when his bag ripped and his possessions (and Edd) toppled out.