The crumhorn is a double-reed wind-cap instrument. This means that the two reeds are enclosed in a rigid cap. The player blows through a hole in one end of the cap, which makes the reed vibrate unimpeded, since there is no direct contact with the lips. The crumhorn is a cylindrically bored instrument, normally made of maple with a curved lower section.
Pitch and compass
Since the crumhorn uses a wind-cap, it cannot overblown. The instrument's compass, therefore, is limited to about a ninth unless keys are provided to extend it downwards. It is possible to use a technique called underblowing, which can extend the range downwards by a perfect ninth.
Because it could not be overblown, a number of different crumhorns were necessary to cover a useful range of pitches. From the soprano to the extended great bass, every pitch from G' to d" was covered. The narrow range of the crumhorn meant that a set of instruments was needed to play even simple music, and its manner of construction, which involved bending while damp to achieve the hockey-stick curve, made it a relatively expensive instrument, so it was largely restricted to professional court musicians.