Sorry all, it's late and I am clashing really hard with the practice book right now. When I have a G clef and a Db as keynote. And a perfect 12th is asked for, in my opinion that would be an Ab in the 2nd octava, right?

Or another example, if I have a G# and a C# written as a harmonic intervall, I can assume that the scale at least includeds C# and F#. So, when I count that would be a perfect 11th? Or am I completely off here?

Yes to both.

Thank you so much!

Assuming your G#is the lower note, yes, that's right.

But the bit about the scale is entirely irrelevant, actually. If you had a piece written in the key of Eb, it would still be a perfect 11th (do they still equally allow compound perfect 4th, which in some ways is easier to understand?)

G anything up to C anything will be an 11th (or a 4th of course) even if it's G double flat to C double sharp or the other way around - obviously that's a silly example as you would never find it in a musical context unless someone was trying to be cussed/clever. The key bears no relation to what the interval is called when it's written.

If you played those two notes on an instrument and said they were in the key of Eb, they would propbably be Ab to Db, with the Db being accidental to the key, but here you seem to be talking about written intervals.

Is the question phrased in such a way as to suggest that the key has sme bearing on what the inyerval is? If so, I wonder why.

Yes, compound perfect 4th is equally as good. I teach my pupils both names, but I feel that using the compound name is easier for the purposes of the exam.

I think it's also worth mentioning that the clef is irrelevant, too (assuming you've read the notes correctly in the first place).

Well, the clef is irrelevant in guaging the NUMBEr of the interval, but you still have to be careful you haven't read, say, F# to B# - in other words, as you say, you still need to read the notes in that clef.

That's what I meant!