Positive organs typically exhibit few stops due to their small size and portable nature; a specification of 8′, 4′, and 2′ flutes is common. Some positives may also have a 2-2/3′ stop, and some have an 8′ reed stop (such as a regal). More complex examples feature a divided keyboard, which allows each stop to be activated separately in the treble and bass portions of the keyboard. This makes it possible to play a melody and an accompaniment simultaneously on different registrations.
The positive organ differs from the portative organ in that it is larger and is not played while strapped at a right angle to the performer's body. It also has a larger keyboard (typically 49 notes or more), while a portative may have as few as 12 or 13 notes. The positive is also not to be confused with the regal, a small keyboard instrument that contains short-length reed pipes.
Positive organs were used at many kinds of civil and religious functions. They were used in the homes and chapels of the rich, at banquets and court events, in choirs and music schools, and in the small orchestras of Jacopo Peri and Claudio Monteverdi at the dawn of the musical drama or opera.
A well-known instance of an early positive or portable organ of the 4th century occurs on the obelisk erected to the memory of Theodosius I on his death in A.D. 395. Among the illuminated manuscripts of the British Museum there are many miniatures representing interesting varieties of the portable organ of the Middle Ages, including Add. MS. 29902 (fol. 6), Add. MS. 27695b (fol. 13), and Cotton MS. Tiberius A VII. fol. 104d., all of the 14th century, and Add. MS. 28962 and Add. MS. 17280, both of the 15th century.
In England they became known as chair organs, later to be corrupted into the "Choir" division found on modern organs.