I am composing a piece for full orchestra and want to make sure that my parts are conventional. I know that there are typically four horn parts, but are 1 and 2 high parts, 1 and 3, or something else?
Conventionally, 1 and 3 take higher parts and 2 and 4 take lower parts. In many scores, horns 1&3 are given one staff and then horns 2&4 are given the next staff. I prefer that method, but you will see plenty of scores that group 1&2 and 3&4—however it will still generally be the case that the odd numbers have higher parts and the evens have lower. Of course, when you make parts, all of this should be separated out, and there should be individual parts for each horn.
The reason is historical: horns 1&2 used to be in a different transposition that 3&4, and composers tended to have a high and a low horn in each transposition. For example, horns 1 and 2 might be in F, and 1 would take higher notes than 2, while horns 3 and 4 might be in G, with 3 taking the higher notes. Modern horns are always in F, but 1&3 taking higher parts than 2&4 has stuck around.
I am creating separate parts for each horn. For future reference though, when you say one staff for each pair of horns, do you mean one treble and one bass clef or just two treble staffs with paired parts? Jul 18, 2017 at 0:54
1Clef choice depends entirely on the range you're writing in. Both staves are often treble, but one or both could be bass if the part calls for it. Of course, since 2&4 tend to have lower parts, they're more likely to use bass clef. Jul 18, 2017 at 0:56
2That said, bass clef is not nor should be the "default" and only used when absolutely necessary. 4th horn parts are quite comfortable reading 4 ledger lines below the treble staff, though beyond that, bass clef, depending on the context, may prove to be more useful. Jul 18, 2017 at 1:19
3@jjmusicnotes Yes, and, while we're on the subject, the old-school practice of writing transposed notes a fourth below sounding pitches when in bass clef instead of a fifth above sounding pitch should no longer be used. Jul 18, 2017 at 1:25
1@PatMuchmore Funny you mention that; I debated including it in my comment but decided I didn't want to worry about confusing the OP. Jul 18, 2017 at 1:27
1 high, 2 low - and you can add 3/4 and 5/6, even 7/8 (e.g. Mahler), but always high/low, always in pairs. Solos often go to 1, but can also be covered by 3/5/7 etc.
It might help to think of each pair of horns as a separate group, like pair of woodwind instruments. You can just add more pairs and occasionally have them all play together. Often the 3/4 horns are doing something completely different in rhythm or key. It's also perfectly OK to use only 2 horns.