The ability to recognize tonal relationships, key changes, modulations, phrases and the like is really what helps make up the entire “universe” of being able to listen and decipher music in a truly confident and professional way.
One of the first things I always ask a new student, especially one who says that he or she is “advanced” is something akin to “if I played or sang you a note, would you be able to recognize other notes in relation to it?” Most folks respond with “kind of.” What this means is that they in fact might have a very good ear, but that they haven’t gotten into really learning what relative pitch truly is, and they haven’t started recognizing chord and note relationships.
Since I had to teach myself and learned in front of audiences and in recording studios at a very young age, I had to really understand that music and learning what things sounded like was going to mean my survival. Of course, I must’ve had a good ear to begin with, but the honing of this “ear training” skill really started to take shape almost immediately. I mean, once I knew The Byrds were playing a suspended 4th version of a chord, I certainly never forgot how to identify that kind of tonality. And it also became just as easy to find that note in relation to every chord form, and every chord shape. And then what does that end up teaching you? It shows you exactly where the all-important minor or major 3rd is within that chord, since the suspended 4th is simply the next note to follow the 3rd!
It’s this kind of “relative pitch” that you must develop if you ever want to be the “real thing” when it comes to music. For example, I of course also had a very firm foundation in playing blues at a very early age, and the I-IV-V was the predominant style of Blues to begin with. Well, of course that totally imprinted the sound and relationships of those 3 tonalities to each other, which in turn right away allowed me to observe and recognize “new” blues progressions such as the I-VI-II-V, or the I-IV-I-VI-II-V-I.
The main thing to recognize is that once you have some of these “ear essentials” in place, you should be able to “by default” and by the process of elimination, continue to expand your ear knowledge very rapidly. In fact, if you really get “the knowledge” as I like to call it, anything will be possible, and you will find your “ear recognition” to be literally infinite!