does anyone know what keys trumpet and French horn are in? I need to transpose some pieces I wrote. Also any top tips for what these instruments can and cant do? I did not write for them before! thanks!

All you need to know about brass instruments (or indeed, other instruments too) can be found here .

About the pitches though: The horn is indeed in F, but the C trumpet described here is the common orchestral trumpet. Though, everywhere else (amateur players, jazz, wind bands etc.), the B-flat trumpet is the standard instrument (the written range is the same as that of the C trumpet, but sounds a major 2nd lower). Unless you know that the trumpetist you're writing for owns a C trumpet, assume that he has a B-flat only.

Two things to add;

1. For horn your written pitch should be a 5th higher than the actual sound.

2. Trumpet players are used to playing c parts on Bb instruments and professionals can and will transpose on the fly. It would probably be best to be very familiar with the players performing and their preferences. They may prefer a C part.

Steve is right. Good players are very good at transposing. However, I've never met one that actually preferred a C part. I'd go the extra mile and transpose. They'll appreciate the effort.

Thank you very much for all that, very helpful

Keep in mind that most pianists are only comfortable reading treble and bass clef. Familiarizing yourself with, then gaining proficiency with the use of envisioning movable "K" clefs will make the transposition of any treble or base clef staffed music very simple to transpose to other keys.

A "K" clef placed anywhere on the staff fixes the line or space of the staff (or above and below the staff as well) as middle "C" at the piano.

As a trumpet player, I was about to toss in that comment. Transposing isn't that difficult, but it's a pain in the arse. I absolutely hate doing it when playing. Make the player's life easier and write in the correct key. There's absolutely no reason not to.