Open (organ stop)
Organ stop whose pipes are not closed at the top (opposite: stopped).
What it means
Open (Latin apertus) is the name for any organ stop whose pipes are not closed at the upper end. The counterpart is the stopped stops, whose pipes are closed at the top by a cap. The difference determines the tone and pitch of the pipe.
An open pipe of length L sounds an octave higher than a stopped pipe of the same length. In addition its tone contains all the overtones of the series, whereas the stopped pipe produces mainly the odd-numbered overtones.
Musical use
Open stops are the basis of organ sound: the principals, the backbone of every organ, are built open and sound bright, clear and carrying. Many flutes are open too. Stopped stops such as gedackt or bourdon sound softer and darker and are often used for accompaniment or solo voices.
Open should be distinguished from stopped and from half-stopped: in the half-stopped pipe the cap is fitted with a small tube, which places the tone between the two constructions.
Performance practice
When registering, you combine open and stopped voices for different tone colors. Open stops supply brilliance and clarity, stopped ones warmth; listen to the mixture in the room and adjust it.