This is an intriguing instrument with strings that are played when the instrument is placed outside or in an open window. As wind passes over them, an ethereal sound is produced with constantly changing harmonics. The largest Aeolian harps are up to three feet in height, allowing for up to a dozen or so gut or wire strings. Some rare versions include a second set of strings tuned an octave lower. It is still not fully understood just how the sound is produced. The earliest Aeolian harps were built in the 17th century.