One of the great orchestral works featuring the horn is the Symphony No. 5 of Mahler. The full work itself has six horn parts. However, the Scherzo movement only has five horn parts, divided as follows in the score:
I actually wrote a paper about Mahler 5 as an undergrad (a great topic!) and have been clear about the number of horn parts my entire professional life. I have performed the work at least five concert series as well; once on principal horn, once on second, twice on third, and this past week again on sixth horn with the Phoenix Symphony (Bravo to the entire section! And to all horn sections of performances past; many good memories).
The soloistic Corno Obligato part is well known in the horn community. What is not well know is that actually the standard set of hand manuscript parts has a “horn V” part in this movement as well. Note the score seen above again; there is no horn V. The printed horn V part is actually just the third horn part copied out again in that same hand manuscript. I suspect it is in the fifth horn part for convenience only.
On this movement normally the first horn will play the Corno Obligato part, the third horn will play first horn, and the fifth horn play third horn. With the sixth horn chilling, tacet on this movement, and the assistant either not playing or only very lightly used.
The reason this is all post-worthy is that I am told that sometimes groups will actually play the horn V part in this movement, having the sixth horn read it. This is not what Mahler intended; don’t do it! It will throw the orchestration off to double one of the horn parts like that. If you are in doubt check a score, which makes it crystal clear that this movement really has only five horn parts, orchestral horns 1-4 and the famous Corno Obligato solo.