(Projoy) What I can't understand is why everybody hasn't got it from birth. As I said, it's only a form of memory, like remembering colours. (Dujon) Most people can tell if someone is out of tune, but only relative to some other pitch which is assumed correct. Most people who play a guitar can tune it so that the instrument as a whole is in tune. I have a friend who plays a few tunes (only at home), and plays rather well, and his guitar is in tune with itself but at least three semitones flat. He has a good sense of tuning but none whatever of absolute pitch. I tuned it up for him but he said the strings were far too tight so back down it went.

[Rosie] OK. Why can't you understand that? (PS. I hate to point it out for a third time, but we've already discussed twice why it isn't like remembering colors! Perhaps you remain unpersuaded?)

[Rosie] Perhaps I can help. I have played guitar and sung in very amateur choirs. so I'm not totally lacking in the ability to hear pitch. But I hear lots of other things at the same time. For example, when tuning a guitar I'm often not sure if two strings are at the same pitch because of the different texture of the sound caused by the make-up and length of the string.
The same applies with other occasions. If a man and a woman are singing a duet I could tell you if there was a discord (to my ears), but would struggle to say if they were singing the same note (either an octave apart or exactly) because I cannot separate the pitch of a sound easily from the rest of its character.
Also I find it easy to sing a tune (relative pitch), but have no idea whether I'm singing the same notes as on a previous occasion. It seems to me that the ability to keep or hear a tune in your head does not imply that you remember the actual sound (which would imply that perfect pitch should be possible), but rather that you remember a pattern, which you can then apply to the sound when you remember or sing it.

[INJ] Perfectly described. It is the same for me, except that I'm usually better at the octave thing. At a weekly music school, I always impressed the teacher by being extremely quick at identifying intervals, and thus have a good memory for melody in the way you describe. I now find I have to make more cognitive effort to recognise intervals, but then I have learned a lot more about the subtleties of language intonation since then (through drama but also through acquiring more social skills). Which is interesting.

To me, anyway.

(INJ) Remembering a whole tune would seem to be much more complex than remembering the pitch of a note yet we seems to do it rather well, just as we remember faces rather than any specific physical feature of them. But to me pitch of a tune is an integral part of it and if I hum a tune to myself in the wrong key (because it's easier, say) I still have to assign a key to it.

...is lobbing an accordion into a skip without touching the sides.

On a more serious note, I've had perfect pitch since I could ever remember, both in the absolute sense and the relative sense. So I have very little trouble transposing at sight, as well as playing at sight, or keeping in tune when singing a song in different keys. I've never known how I do it - I also have a pretty fast learning memory for music too. But, oddly enough, I'm lousy at matching names to faces.