Stops are typically classified into three main categories:
1. Flues: Flues are pipes that produce sound by the vibration of air passing through a flue, which is a narrow opening at the top of the pipe. Flues are the most common type of stop, and they come in a wide variety of shapes and sizes, producing different tones.
2. Reeds: Reeds are pipes that produce sound by the vibration of a thin metal tongue, called a reed, against a metal frame. Reeds produce a more penetrating and vibrant sound than flues.
3. Mixtures: Mixtures are stops that consist of several pipes of different pitches that sound together when the stop is engaged. Mixtures add fullness and complexity to the organ sound.
In addition to these main categories, there are also a number of other types of stops, including:
* Solo stops: Solo stops are stops that are designed to be played as solos, and they are typically louder and more expressive than other stops.
* Accompaniment stops: Accompaniment stops are stops that are designed to provide a supportive background for other stops, and they are typically softer and more subdued in tone.
* Percussion stops: Percussion stops produce sounds that resemble percussion instruments, such as drums, cymbals, and bells.
* Special effects stops: Special effects stops produce a variety of unusual and interesting sounds, such as bird songs, wind chimes, and train whistles.
Organists use the combination of different stops to create a wide variety of sounds and effects, making the pipe organ one of the most versatile instruments in the world.