In classical music scores we often find French horns of different pitches. Why is that?

Question

For example, in many pieces, there are two horns in F and two in E flat. What is the reason? Divide the horns to be able to give each group the better matching parts concerning intonation? For pitch range reasons? For fuller sound in certain parts?

So, why are there not simply four horns in F, but two in F and two in E flat?

And when and why do we use different groups of horns in which pitch when we orchestrate a piece of music?

An example can be found here: http://imslp.nl/imglnks/usimg/7/7a/IMSLP273434-PMLP01572-II._Erster_Fassung.pdf

Possibly in today's musical practice and in modern scores these different groups of horns are no longer necessary due to technical improvements.

In Mahler's 6th there are, however, trumpets in B♭ and trumpets in F. This would be another example for my question.

Answer 1

I'm not entirely certain of whether this is providing information that the OP doesn't already have. Apologies if this is a repeat of known knowledge.

In the early evolution of the horn as an orchestral instrument, it had no keys, so the player could only play notes in the harmonic series of the instruments, plus-or-minus hand-stopping (which changes the pitch of a given instrument by a fixed amount) and whatever amount of "bending" the player could achieve with their lips. In higher ranges the harmonic series gets close enough together that the player can actually manage the full scale, but at lower ranges that's impossible and in the mid-range it makes certain notes or runs very difficult to play quickly or with exact tuning (and impossible to get both!)

Instead of keys, the horn came with a set of crooks, which the player could substitute in (at the cost of a few 10s of seconds) to change the key of the entire instrument.

You couldn't do this in the middle of a piece but you could certainly do it between concerts, pieces, or movements.

So if, as composer, you want horn notes that aren't compatible with a single pitching, within one piece. Then you are obliged to get multiple horn players each with a different crook, and thus in a different key.

So, ultimately: "to be able to give each group the better matching parts concerning intonation?" is correct, but with the emphasis on "it might not actually be realistically POSSIBLE" rather than "it might not sound as nice".

Nowadays the concept is of course completely obsolete, but the remnants of the instrument's history remains. (And it gives horn players an excuse for why they were playing the wrong notes: "Oh, oops, I forgot it was transposed" :D )

Answer 2

One possible explanation I finally found comes and cited from https://www.vsl.co.at/en/Horn_in_F/Notation:

"In classical symphonic music, a pair of horns was generally used for pieces in a major key, whereas two pairs were used for pieces in a minor key. This was done for harmonic reasons, since it was the only way to produce the second subject in the parallel major key.

Classical sonata movements really consist of two “themes” or “subjects”. These can be compared to two more or less active characters. The first “theme” is of course always in the principal key (tonic), the second in the most closely related, which is the one a fifth above it, the dominant. This applies to pieces in a major key. In pieces in a minor key the second theme is a third higher, in the parallel major key. But because the dominant in minor is in major on account of the leading note a “chromatic conflict” arises which could only be solved by one or two pairs of natural horns in different pitches. One of the horn’s main tasks was to support all the harmonically relevant tones (see for example W. A. Mozart, Symphony in G Minor, K.V.550)."

Naturally, I am furtherly interested in any other reasons for the item questioned.

Answer 3

In the score you referenced, the first and last movements are in G minor. Since the valveless horns of the classical period only played the notes of the natural harmonic series, a horn in G could only play B natural, not B flat. It was common to use another pair of horns in B flat to provide that important note in the minor scale, and they could also play other useful notes like F natural.

The second movement is in E flat major, so the single pair of horns are in that key.

In the third movement, the minuet is in G minor but the trio is in G major. The horns (in G) are used as solo instruments in the trio, and Mozart doesn't need any B flats in the minuet, so the other pair of horns get a rest.

Two horns are can make more than enough noise to be in balance with a classical-sized orchestra, and four horns all in the same key were only used for "special effects" requiring an over-sized brass section.

Note, all this only applies directly to classical orchestral horn parts. In the baroque era, horn parts were often written higher, where the instrument can play a complete diatonic scale. By the 19th century, horn technique had reached the point where valveless horns could play a chromatic scale over almost the full range of the instrument, though the tone of the notes was different depending how far they were "bent" from the natural harmonic series, so the natural key of the horns was still chosen to correspond with the key of the music. Composers like Brahms considered that valved horns were inferior instruments with poorer tone quality, and wrote chromatic horn parts for valveless instruments.

Answer 4

It indicates the nature of the transposing instrument. It indicates the interval by telling you what note sounds when you play middle C. The French Horn in F sounds a Perfect Fifth lower than notated while the Bb instrument sound a Major Second below notation.

Remember that the transposition is usually done down from the note C. Some of the notable exceptions are Eb Clarinet and Eb Trumpet and Soprano Cornet / Sax which are sound minor thirds higher than notated.