How is pitch determined in free-reed instruments such as harmonicas and accordions?

In free-reed instruments such as harmonicas and accordions, the pitch is determined by the physical characteristics of the free reed itself.21

Specifically, the pitch of each individual reed is primarily determined by its length and thickness. Because these instruments produce sound vibrations in the open air rather than within an enclosed air column (like a pipe), the reed acts as the sole source of the pitch. While the reed's length is the primary factor, tuning can also be accomplished by weighting or thinning the metal tongue of the reed.23

Unlike woodwind instruments such as the clarinet or oboe, where the length of the air column within the instrument's body determines the pitch, free-reed instruments do not require a resonator to modify the pitch. The reed vibrates freely through a closely fitting frame, and its frequency of vibration—and thus the resulting pitch—is established by its own mechanical properties.23