Is it "legal" for a conductor to reduce number of instruments in a piece?

Question

Is it a common practice for a conductor to reduce the number of instruments? I have an example here:

Wikipedia says that this piece by Mozart was written for 4 horns, but in the video we see only two.

Answer 1

There are definitely four horns in that video. You can see them in the wide shots just left of the timpani (stage right). You can clearly see them (well, blurredly) raise their instruments in preparation for their entrance in measure 5. Another clue that there really are four horns is that they're playing natural horns, and it would not be possible to play the notes we hear in the video with two natural horns.

To answer the question, it's not common practice at the professional level to omit instruments, though it isn't unheard of. It would typically be done for a specific reason and with some discussion in the program notes (for example to "recreate" a specific performance that was known to have taken place with reduced forces). Adding instruments or substituting instruments is also fairly uncommon, though any of these would be more likely outside the professional context, for example with a student or community orchestra.

Answer 2

There is nothing legal or illegal about this. It is not a question of law. You could ask if it is legitimate for a conductor to change the number of instruments use. That legitimacy would most likely be in regard to period practice or instrumentation indicated in a score. Another term to use is authentic, as in "authentic performance." But before getting into that you need to know about the classical period and whether an exact number of instruments was specified in the score.

The example you give shows that at least four horns would be need to cover the four part harmony, but beyond that the score just says "horns", just simply plural, not a specific number.

One thing to consider, at least from a historical perspective, was where a work was composed and if it was composed for a specific orchestra. That could give some indication of the intended orchestral forces. In the classical era the exact number of instruments used depended on what orchestra was performing. In the case of K. 183, the only thing I could confirm with a few web searches was it was composed in Salzburg, but there isn't any record of its first performance. It seem that it probably wasn't even performed during Mozart's lifetime.

I don't see the point of quibbling about the exact number of instruments in a modern performance for classical works that did not give exact instruments counts and might not have even been performed in a composer's lifetime. How can you confirm exact numbers in such cases?

FWIW, comments suggest you've miscounted the number of horns in the linked Youtube video.

Answer 3

The short answer to the question is no - there is nothing illegal about changing the instrumentation for music that has gone out of copyright. *)

And for older music it is done in just about every performance of music written about 1930 or older. The reason is that instruments used now are not the instruments the music was written for. As examples:

• Before 1930 or so English composers wrote for small-bore trombones and French bassoons. Both types are different from the instruments most often used today. The German bassoon is a mid-1800s invention and only slowly moved into some countries -- as an example Stravinsky wrote for the French bassoon.
• The valved horn, the valved trumpet, the tuba and so on are all inventions from circa 1820. Before that valveless horns and trumpets where used. The parts played by the tuba could be written for ophicleide or even serpent (Symphonie fantastique has one Serpent and one Ophicleide, rarely seen in current performances). And the French tuba (high C) that for example Ravel wrote for is very different from the large CC tuba used in most orchestras today, closer to what we today call euphonium in size.
• The violins we play on today with steel strings are quite different from older types. (As a side note, all the Stradivarius instruments have been heavily modified to allow for the higher string tension and longer finger board).
• and so on ...

There are orchestras / performances done on period instruments and they sound quite different from modern instruments.

Additionally, most composers wrote for the orchestras available and probably adapted to what was at hand in the amount of players. I believe that very often contemporary performances had adapted settings. Of course, there were (are) exceptions, where the composer very specifically asked for certain instruments. Examples include Wagner and his requirement for the so-called Wagner horns.

*) There could be specific license requirements for music or editions that are still in copyright, but that is a different question.