Quote from: Le.Tromboniste on Apr 16, 2017, 04:55PMYup it's technically a sharp 3rd because D is the 11th harmonic of Ab, not A, but given that the 11th harmonic is virtually a quarter tone flat, it's pretty much exactly in the middle.
That´s right. In Europe it´s flat 2nd in USA sharp 3rd, the same thing really. If you do messure it is a tad closer to 2nd, does not matter, it is in the middle.
(Like low Eb on a attachment is known as flat 3rd or sharp 4th. Closer to open 3rd though, one reason for students (and som pro) to play it sharp.)
On some horns the D is good on first, some horns better on b2 (#3) some on 4, and some horns not at all.