Ah … the Pipe Organ; the King of Instruments, so called by Mozart.
Indeed, the Pipe Organ is not only a large physical instrument–sometimes weighing several dozens of tons — but they also produce an equally large sound; and according to my taste, they produce quite powerful, beautiful, and dignified sounds.
But the Pipe Organ wasn’t always a colossus. The instrument has an origin in antiquity, specifically Greece, where it was no larger than a dresser, one manual, and powered by water and gravity. Water would sit in a chamber, and when a key was pressed, the water would fall by gravity down a pipe mechanism, forcing air through a reed, producing a sound.
Modern organs are powered by the same principle, except in Bach’s day, the air was collected in a wind chest, and supplied by foot billows, which had to be pumped by an assistant; and with today’s organs, an electric fan or some electric arrangement supplies air to the wind chest.
An organ stop is a device which engages certain pipes, which produce certain sounds. Some large organs have dozens of stops, and therefore sounds; like the Principle stops, woodwind stops, horn stops, and so on.
In terms of sound variety, and dynamism — loud or soft — the organ is an entire orchestra in one instrument.
One of the unique features of an organ is the foot manual — the foot pedals. A talented organist can play three, four, and even five lines of counterpoint at a time, by dividing the lines between the hands and feet. Bach was such a master at the foot manual that he could play a complicated sequence with his feet what others could not play with their hands.
There is so much more on this topic to discuss, but I have to limit myself.
Here is a recommendation for entry listening:
Try Bach’s Fugue in G major, BWV 577.
While listening, follow the subject as it travels from tenor to alto, then to soprano, and concluding — gloriously — in bass; this section being played by the feet.
Or, try Mozart’s Organ Fantasie in F minor.
This is the Grand Organ of St. Jacob’s Cathedral in Austria, for example.