This lesson covers one of my favorite topics--the stops of the organ. Today we'll cover the nitty-gritty of the stop numbers and Roman numerals, and next lesson we'll cover the beauty of the different tonal families.
Before we begin, however, it's important for you to familiarize yourself with the action of your stops. Some stops are simple tabs, like my older home organ. You push them down to select a stop, then lift them back up to deselect a stop:
Other organs may have rocker tabs or draw knobs. Most rocker tabs are selected by pushing on the bottom of the tab and deselected by pushing on the top, however some will select or deselect in either place. Draw knobs are pulled out to select a stop and pushed in to deselect a stop. Your organ may function differently, so take the time and learn how your organ works.
Once you are familiar with your organ's stop action, it's time to pull out the stop list you compiled in Before We Begin: Acquiring the Essentials. You'll notice that in addition to the names of the stops, two different kinds of numbers are also present. Almost every stop has a number such as 16, 8, or 2 2/3. Some stops may also have a Roman numeral, such as II or IV.
All electronic organs herald back to pipe organs and lengths of pipe. A rank of pipes with the longest pipe being 8 feet long will sound at unison pitch, like a piano. 8' stops are the foundation of organ playing. A rank of pipes with the longest pipe being 4 feet long will sound an octave higher than a piano. (The exception to this rule is a rank of stopped pipes, where the lowest pipe is 4 feet in length but sounds at unison pitch, so it is known as an 8' stop. Fortunately, you don't need to worry about stopped pipes--just the numbers on the stops which tell us the pitch they sound.)
The lowest stop on some church organs is a 32' stop. This stop sounds two octaves below the 8' foundation and is found in the pedal division. While the 32' stop adds powerful, rich bass tones to a full organ, it should rarely be used in hymn accompaniment.
The 16' stop, which sounds an octave below unison, is the foundation of the pedals in congregational accompaniment. When paired with an 8' stop, the 16' provides the substantial bass sound the organ is known for.
As mentioned before, the 8' stop is the foundation of the organ, as this "unison pitch," or fundamental pitch, is in the same octave as the piano. However, it must be paired with a 4' principal or flute stop in order to support congregational singing.
The 4' stops sound an octave higher than the fundamental pitch.
Yes, there is a pattern here! A 2' stop is two octaves higher than the fundamental pitch, and a 1' stop, which you may or may not see on your organ, sounds three octaves higher.
Along with these stops that represent various octaves, are mutation stops with fractions that produce non-octave tones of the harmonic series. Stops with 3 as the denominator of the fraction always produce a perfect fifth above the fundamental note, so 2 2/3’ produces a pitch that is an octave (since it's shorter than 4' but longer than 2') and a fifth higher than the fundamental pitch. 1 1/3' is two octaves (shorter than 2' but longer than 1') and a perfect fifth above the fundamental pitch. Stops with a 5 as the denominator will produce a major third above the fundamental, so 1-3/5’ sounds two octaves and a major third above fundamental pitch.
Here's a visual from the Pipe Organ Education Project that may help with understanding mutations:
Since mutations feature a tone between the octave-sounding pitches, their addition adds an additional timbre to the bright, full organ sound. They are also useful in building solo registration when coupled with 8' stops. We'll discuss them more when we cover various registrations.
Roman numerals mean a very different thing than the numbers we just covered. Roman numerals represent compound stops, or stops that pull more than one rank, or simulated rank, of pipes. The Roman numeral tells you how many ranks of pipes sound on that particular stop. If it's II, two ranks sound. If it's IV, four ranks sound.
A common compound stop is a Flute Celeste II 8'. A celeste rank is pitched slightly higher than a regular rank. Combining it with a rank of the same family (such as Viole 8' and Viole Celeste 8') produces a warmer sound similar to the string section of the orchestra where all the players are using vibrato. This Flute Celeste II 8' stop will pull both a Flute 8' and a Flute Celeste 8'.
Another common compound stop is a Mixture IV, which includes four different principal ranks. Most mixture stops include only octave and fifth pitch levels, such as 2 2/3′, 2′, 1 1/3′, and 1′. (Often the pitches in a mixture may change, or break, depending on where you play the keyboard. Breaking simply means a higher pitch drops out and is replaced with a lower pitch.)
At your organ with your stop list in hand, play middle C while selecting different stops. Make notes next to the stops on your list if you like. Familiarize yourself with your mutations and compound stops. In Lesson 3: Demystifying the Organ Stops, Part 2 we will cover the different organ tonal families and there will be a lot of listening hands-on activities which continue into Lesson 4.