The portative organ lacks a reservoir to retain a supply of wind, thus it will only produce sound while the bellows are being operated.
The pipes are arranged on a small rectangular windchest and supplied with wind by one or two bellows placed at the back, or under the instrument.
[3] In principle, the portative is a smaller instrument than the positive organ, which features more ranks of pipes and a larger keyboard.
[3] Medieval portative organs, so extensively used during the 14th and 15th centuries, were revivals of those used by the Romans, of which a specimen excavated at Pompeii in 1876 is preserved in the Museo Archeologico Nazionale Napoli.
[3] The small instrument that is often taken for a syrinx on a contorniate medallion of Sallust in the Cabinet Impérial de France in Paris may be meant for a miniature portative.