I consider my understanding of theory pretty basic. My school discontinued its commercial music program the semester I started. I made it through the first three semesters of classical theory, which basically covers major and minor diatonic theory and associated 7th chords. We also had one chapter on augmented 6th chords, which I have forgotten. AFAIK the augmented 6th chords don't typically relate to jazz theory...or if they do I never made the connection.
I would say I have harmonization for major keys down pretty well. Never got solid on harmonization for the minor forms. I know my major and minor 7th and 6th chords really well. I would say I am less solid with augmented and diminished 7th chords, but I generally don't have any trouble handling them when they occur in music.
While going to school, I bought a Real Book and started learning to interpret chord symbols and apply theory to improvise bass lines in real time. Before entering college, I had played in a university big for a couple of years while I was still in high school. About half of the tunes we played used lead sheets with chord notation. The instructor for the class gave me recording and the lead sheets, and I learned to play along with the recordings well enough to fake it.
If the harmony of a song is mostly diatonic I can sight read chords pretty well. However, the more altered the harmony is, the more I have to stop and work things out. Also the more limited my bass lines tend to get. When a bunch of altered chords are thrown together, I may only be able to work out one or two lines that I think work well.
I have practiced the basic modes intermittently, and sometimes apply them. I have a hard time keeping them committed to memory. I am aware of and understand the underpinnings of the modes associated with the harmonic and jazz melodic scales, but consider it a lost cause to memorize them.
I have also put some fairly minimal effort towards learning to use relative pentatonic scales to play outside. At this point I have forgot most of the formulas I was working on.
I have never studied any higher-level approaches or systems, or common scales a real jazzer would probably know. I can sort of feel my way through the whole tone scale or the two diminished scales, but only as finger patterns. I have never really tried to do them in my head using note names. I wouldn't know a bebop scale if it bit me on the ***.
Part of the issue is my mind does well memorizing some types of information, but I feel like I have a memory deficit when it comes to retaining anything beyond basic theory. Part of it is probably my lack of discipline or some sort of attention deficit disorder.
Overall I seem to struggle with numbers and higher levels of spatial reasoning. I feel theory is made up of mostly very simple numerical concepts which are integrated with spatial reasoning, but my working memory is not large enough to hold and juggle enough datapoints to work stuff out in my head. There was a time toward the end of my time in college where I started getting a sense that I was near a breakthrough as I was starting to visualize spatial relationships between the notes. Then life happened and I needed a job and left the program. Although I later turned pro, I never again studied theory with enough rigor to get back to this point.
I have a decent ear, but not a great ear. I am most comfortable playing jazz from a lead sheet. I move in and out of different thought processes. I might use my ear for a few bars and then shift and think about it from a theoretical view point...when my ear starts to feel lost, I sort of switch over to theory.
Never had any trouble memorizing several hours of pop music. But I struggle to memorize jazz. I have memorized some of the more simple jazz standards, but I tend to get lost in more complex changes. The way I memorize songs is sort of like learning to anticipate what sound comes next. I use very little theory when playing by ear. With jazz I have a tendency to get pulled up into the harmony where I get lost. This is especially true when dealing with non-functional chords.
Overall I still think I was blessed with above average musical talent overall. That's not really as arrogant as it sounds. Consider many people seem to have no musical talent whatsoever. Also my level of talent is nowhere near the top of the musician pool. Great players did not seek me out. But I was generally competent and solid as long as I had some time to prepare, or was playing something I had played extensively.
Also my strength was versatility rather than specializing. Musicians in the organizations I worked in were expected to play classical, jazz, and pop at relatively high level. I was not as good as most of my peers when they were playing within their specialty, but I think I was generally better across all genres when they were playing outside of their specialty. I would say I was valued and successful as a player.
I had the impression most of my peers who specialized in classical had totally flushed all of their knowledge of harmony. They could play scales, but most struggled to even spell basic 7th chords. They had no concept of how to approach improvisation. Some were forced into trying and really struggled.
