Last Tuesdagy my two teachers and I spent some time sight reading all sorts of scores they happened to have around, and one of the pieces was a chorale by Ingegnieri. It was short and easy, but I cannot figure out what key it is in? The key signature contains a b-flat, which to me indicate F-major or D-minor. However, the piece starts and ends on G, and all F's have an accidental and become F-sharps. (Both when going up and down, which excludes D-minor. Additionally, C's have no accidentals.) Major/minor tonality apparenquitly did not become ubiquitous until the 17th century, and since Ingegniery lived 1545-1592 I assume that the piece might be in some older kind of scale/mode. But how do I find out?

The final chord is "G- B natural - G", which also makes me wonder why a B-natural here, when all other B's are B-flat?

Anyone who can help me out?

Without seeing or hearing the piece, but going on what you wrote, it sounds like the piece is actually in the key of G minor. In some minor scales, the 6th and/or 7th note are raised a half step (semitone)which would explain the F#. The final chord would then be a Picardy Third (Tierce de Picardie) where the third of the expected minor triad is raised a half step (semitone) to create a major triad, as a form of resolution.

Yes, that seems to be pretty close. But in G minor the E:s should be flat as well, and this is not the case in the chorale...

The tonality we all learn in our music theory classes began in the Baroque period. Renaissance (and medieval) tonality follows different rules. I haven't had the opportunity to learn these rules in detail, so I can't help you any more than that.

A piece of music with Bb flat in the key and where the melody starts and ends on G may be in the old Dorian mode.
I can't speak for the F# accidentals. They actually may have been edited-in at a later point. Unless you are reading from a manuscript, you can't rule out editing. They may have also added the Picardy third (b nat. ) at the end.

That sounds like G Dorian. A lot of Renaissance pieces are in that mode. As for the for the final chord, it sounds like it was just a random key change. This happened a lot in baroque style music (perhaps the most famous example is at the beginning of Toccata and Fugue in D minor).

Many thanks to everyone! I'm learning a lot. Dorian G appears to be correct.

Found the following quote in "Editing early music" Caldwell (2001) through google books:

Baroque key-signatures, however, are often archaic in the sense that they do not correspond to the modern conventions for indicating the minor or major key of the music for which they are being used. Thus we have, e.g., one flat fewer or one sharp more for minor key music (supposedly representing the Dorian mode and used as late as Haydn), and one flat more or one sharp fewer in major-key music (‘Mixolydian’ notation).

As previous poster says, sometimes composers leave out flats in the key signature, eg. Bach's G minor violin sonata is sometimes printed with 1 flat and one of the toccatas in Dm was originally printed with no flat (and got the nickname Dorian as a result). You may recall doing a scale called a melodic minor and for G minor that would have (ascending) G A Bb C D E F# G and descending G F Eb D C Bb A G. So it is reasonable that a tune may have come into the composer's head that consisted of the notes in the ascending scale. Maybe the composer had a tune come into his head and could never work out what key it was in and just wrote it down thinking that people would prefer to listen to the music rather than worry about what key it was in.

Yes, Philoshite, it's very similar to melidic G minor, but the F's are sharp both when ascending and descending i nthe piece. I am practicing the minor scales right now, and that the piece did not fit was one of the reasons asking about it.

Maybe the composer had a tune come into his head and could never work out what key it was in and just wrote it down thinking that people would prefer to listen to the music rather than worry about what key it was in.

I love that attitude, but personally I enjoy playing around with theory because, as an adult beginner on flute, it's easy to learn compared to all the practical aspects of flute playing I'm learning. Theory is not instead of playing/listening to nice music, it's just an additional aspect that can be quite fun or uselful in the right context.