ShiroKuro So, now I wonder if my understanding of relative pitch is correct.... I used to play guitar (before starting piano, I stopped no long after starting piano). And I could tune my guitar, either by using a tuning fork, or tuning it to another instrument.
I also thought that ability meant I had relative pitch.
I think I understand what you're saying. You're thinking of the word "relative" --- "relate" ---- as relating one source of sound to another source of sound. So hearing the A on the tuning fork, and then tuning your guitar until you hear that same A on that string. This requires a listening skill, because you have to actually be able to hear when they're different and when they're the same pitch. This is not what "relative pitch" means.
Hm, it's how notes relate to each other within the context of musical structure. In Twinkle Twinkle Little Star, in G major it starts G, G, D, D, E, E, D ..... in C major: C, C, G, G, A, A, G ..... in F major: F, F, C, C, D, D, C etc. You will probably hear this as "the same melody" each time, even though the notes are different. One thing that is the same is that each time you to from the Tonic (1) to the Dominant note (5), up a whole tone and back down a whole tone. The melody could be rewritten as 1, 1, 5, 5, 6, 6, 5 for the degrees of the major scale in that key. The notes relate to each other in terms of being a perfect fifth away from the previous note, a whole tone away etc. They also relate to each other in terms of their place within the major scale.
My relative pitch was made stronger due to Solfege being my only reference for decades. I'd hear Twinkle as Do Do So So ... but would have no idea what key it was in. In order to know the key, I'd have to poke the piano until I found the note that sounded the same as the first note I heard.
The person with "absolute" or "perfect" pitch hears a pitch as that pitch. They'll hear that second new note as D or G or C precisely, and do not rely on the ability to hear "up a perfect fifth", i.e. how the 2nd new note relates to the first.
That person can also get quite muddled if they are used to hearing the C# minor prelude in C# minor, and a slowed down recording plays it in C minor, or someone has transposed it. Those with relative pitch will remain unphased, because the Dominant note is still the Dominant note, five over from the Tonic.