The crumhorn is a capped reed instrument. Its construction is similar to that of the chanter of a bagpipe. A double reed is mounted inside a windcap at one end of a long pipe. Blowing into the windcap produces a musical note. The pitch of the note can be varied by opening or closing finger holes along the length of the pipe. One unusual feature of the crumhorn is its shape; the end is bent upwards in a curve resembling the letter 'J'.

Crumhorns make a strong buzzing sound. They have a limited range, usually a major ninth; while it is theoretically possible to get the reed to overblow a twelfth above the fundamental note, this is extremely difficult since the reed is not held in the mouth, so in practice all playing is confined to the fundamental series. Some larger instruments have their range extended downwards by means of additional holes and sliders or by dropping the pressure. Modern instruments have their range extended upwards to an eleventh by two keys. Crumhorns can be chromatically played by using cross-fingerings, except for the minor second above the lowest note.

Because of the limited range, music for crumhorns is usually played by a group of instruments of different sizes and hence at different pitches. Such a group is known as a consort of crumhorns. Crumhorns are built in imitation of the vocal quartet with soprano, alto, tenor and bass as a whole family, as was true with most instruments of the Renaissance. Occasionally some higher and lower sounding instruments are built, but only the great bass has come to stay in addition to the four other sizes.