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The reeds of an early 20th century button accordion, with closeup.

A free reed aerophone is a musical instrument where sound is produced as air flows past a vibrating reed in a frame. Air pressure is typically generated by breath or with a bellows.

The following illustrations depict the type of reed typical of harmonicas, accordions and reed organs as it goes through a cycle of vibration. One side of the reed frame is omitted from the images for clarity; in actuality, the frame surrounds the reed on four sides.

	A reed is fixed by one end in a close-fitting frame.
	Air pressure is applied; the reed prevents air flow, except for a small, high velocity flow at the tip.
	The reed is drawn through the opening, allowing the air to pass.
	The elasticity of the reed forces it back through the frame.

Each time the reed passes through the frame, it interrupts air flow. These rapid, periodic interruptions of the air flow initiate the audible vibrations perceived by the listener.

In a free-reed instrument, it is the physical characteristics of the reed itself, such as mass, length, cross-sectional area and stiffness, that primarily determine the pitch (frequency) of the musical note produced. Of secondary importance to the pitch are the physical dimensions of the chamber in which the reed is fitted, and of the air flow.

The most likely precursor to free reed aerophones is the Jew's harp, an instrument known to many cultures throughout the world, and by many names. In this instrument, the main sound producer is the vibrating reed tongue itself, rather than the air flow.

Various free reed instruments appear to have been invented since antiquity, but were unknown in the West until comparatively recently. Among the ancient instruments, the Khene of Laos and the Sheng of China have survived to modern times. It has been claimed that the Sheng was brought to Saint Petersburg, Russia near the end of the 18th century, inspiring a series of inventions in the early 19th century that were the foundation of the development of the modern free reeds; Cyrill Demian's (see below) patent of 1829 however states that the reeds in his instrument "were known for more than 200 years as Regale, Zungen, Schnarrwerk, in organs."

Some notable early modern free reed instruments:

• The Chinese hulusi and bawu
• Querhammerflügel with Aoline, circa 1810, made by Johann Kasper Schlimbach at Königshofen Bayern, using steel reeds and frames made in one part.
• The hand-aeoline, by Christian Buschmann, 1822.
• The accordion, patented in 1829 by Cyrill Demian.
• The concertina, patented in two forms (perhaps independently):
  • Sir Charles Wheatstone, 1844.
  • Carl Friedrich Uhlig, 1834.

• The harmonica.
• The harmonium.
• The melodica.
• The bandoneón.

In the related woodwind instruments, a vibrating reed is used to set a column of air in vibration within the instrument. In such instruments, the pitch is primarily determined by the effective length of that column of air.

• What Is A Free Reed?
• Classical Free Reeds
• Reeds World Musical Instrument Gallery