Which instruments are most similar to a pipe organ?

* Harmonium: A free-reed organ that produces sound by passing air through a series of reeds.

* Accordion: A free-reed instrument that consists of two wooden boxes connected by a bellows. When the bellows is squeezed, air is forced through the reeds, producing sound.

* Melodeon: A free-reed organ that is similar to an accordion, but with a single, larger bellows.

* Concertina: A free-reed instrument that consists of two hexagonal boxes connected by a bellows. When the bellows is squeezed, air is forced through the reeds, producing sound.