The style of French horn first used in performance, known today as the "single horn," has a single continuous metal tube that connects the three rotary valves of the instrument. The horn player presses the valves with her fingers as she plays to change the notes, keeping her other hand in the wide bell to adjust the pitch and volume. The single French horn is available in two keys: F and a B-flat. B-flat is typically represented as "Bb." The single French horn is the most common style of French horn for students and beginners.
Double French horns were developed some time after single horns to accommodate the desire to have both keys, F and B-flat, available in one instrument. This development gave French horn players access to the entire chromatic scale. It also gave way to a great variety of compositions to take advantage of the French horn's new flexibility, accomplished via the addition of a fourth rotary valve the player depresses with his thumb — known therefore as the "thumb valve." This fourth valve cuts off approximately 4 feet of tubing from the air passing through the tubes, altering the sound and range of the instrument. A double horn can be identified via its thumb valve, located at the top or bottom of the row of valves. The thumb valve is a flat circle without a valve lever, unlike the other three.
A descant French horn is a variation on the double horn, designed for playing at higher octaves. This style of French horn is mechanically similar to a double French horn, but when the fourth valve is engaged, taking the instrument from B-flat to F, the F becomes high F — a full octave shorter than the lower octave F on a standard-style double horn. The descant horn offers even more range to the instrument, and many compositions require this style of "high horn" playing.
Triple French horns are a combination of the descant and double styles. With a fifth valve available, a player can select between B-flat, low-F, the higher-octave F or an E-flat. Triple horns are quite a bit heavier than their double and descant counterparts, and this style of French horn may cause some degradation of the sound quality.