I know this has been asked before, but there wasn’t a lot of consensus as to what the answer was and I haven’t been able to find anything conclusive online, so I’m asking anyways.

I’ve been looking at some fairly easy Scarlatti pieces and am having a hard time realizing the trills. I’ve always been perfectly comfortable doing the usual start-on-the-auxiliary-note approach for guys like Bach, Mozart and even Beethoven, yet with Scarlatti there seems to be a dilemma. Most of what I’ve read on the topic has somewhat aggressively proscribed always starting on the auxiliary note and implied that the principle-note trill is some sort of 19th-century-spawned plague that needs to be wiped out of existence. Yet while looking at a certain simple Sonatina (K 34), my edition (Henle) does not show realizations for the trills, but has fingerings which strongly imply a principle-note execution. Furthermore, the recordings I’ve found unambiguously do trill this way and it seems to fit musically. At first I thought maybe Scarlatti is some bizarre exception to the rule, but in almost every instance I’ve found of someone asking about this online the answer is the formula: Scarlatti = Baroque = auxiliary-note-trill (which is itself contradictory with the frequent insistence that the application of auxiliary-note trills be applied for the classical period as well). This is a strange case of what everyone says you’re supposed to do being one thing, and what everyone actually does (as far as I’ve seen) being the exact opposite.

Has anyone else had this problem? If so, what did you choose to do?

In general, trills in Scarlatti's time started on the auxiliary note but if, in your opinion, it sounds better on the principal note, then by all means, play it that way. (Oh, and if the note preceding the trill is on the auxiliary note, then start on the principal note.) It wasn't until modern times that it became fashionable to stick to the score. Back then, a certain amount of improvisation was expected. Go with what your ears tell you.

My favorite quote on the subject, and I do not remember who said it, said that treatises about music at the time said that trills start on the auxiliary note so often that it must have been almost universal practice not to!

Oh. I feel a little silly now. In this particular sonatina each note preceding the trill was the auxiliary note of the trill. However, now I realize I’ve been blissfully not following that exception in various Bach pieces – that is, playing the trill on the auxiliary note even when it was also the previous note – and it didn’t feel as awkward to do so. Weird. I’m half tempted to go back and “fix” my Bach pieces in those situations now, but probably won’t bother since I’ve gotten comfortable with them and I doubt anyone who hears me would even notice or care that much. I guess I’ll just go with the vague “do what feels right” approach even though it bugs me a bit when it seems like there’s no super-definitive correct way to do something.

Scarlatti is one composer who seemed to go both with the conventions and against the conventions of his times and this makes it sometimes difficult for scholars to agree on what he meant. His music is full of small "oddities" that some editors felt the need to be corrected. In his times the execution of ornaments was a combination of the performers skill/experience and the composers notation and we simply do not know enough to know what he meant sometimes. I would do the general baroque convention, but also apply my own judgment, while still taking into account that if something sounds a bit odd in Scarlatti it may be exactly what he meant it to sound