NB: The espresso pictured above was absolutely delicious. It is for illustrative purposes only.
Back when I started brewing speciality coffee, I learned that the classic sign of underextracted coffee was sourness. Overextracted coffee was usually bitter (although exactly what constitutes overextracted coffee is a rabbit hole in itself) and underextracted coffee was usually sour. However, no matter how many cups of coffee I made with varying grind sizes, I was unable to taste sour notes in my coffee. I detected fruity notes but nothing sour. If I made a cup of coffee that didn't taste great, it usually tasted bitter.
I have a theory for why I didn't taste sourness back when I started brewing speciality coffee at home: my grinder was not good enough (which is largely a product of price—you get what you pay for in grinders).
I don't have any scientific evidence to back up my claim. It is based on anecdotal experience. The more I think about it, the more I am intrigued. I suspect that poor quality grinders may not be able to result in a sour brew—unless you really try to underextract your coffee—because the grind size is not uniform enough. The coffee will be dominated by the bitter compounds that are released by bigger coffee pieces rather than sour compounds released by smaller coffee pieces.
Another idea does come to mind: maybe I did not have a good understanding of what sour was back then.
I think there is some truth to this. I only really learned about sour coffee when I started making espresso at home. That made me realise just how sour coffee could be. My grind size was way off what I needed and my grinder was not built for espresso so I had a limited range in which I could dial in. But even then, using the same grinder, I couldn't get a sour brew when making filter coffee. It was only after I got the Comandante that I started to taste the sourness in coffee. And I like to think that I could distinguish sourness from bitterness before then. 
Again, my theories could be wrong. I haven't read any evidence that suggests cheaper grinders are unable of producing a sour brew. I wonder who would even do that research. Learning more about this topic would help advance our knowledge of grinding coffee as a whole and would maybe even merit a change to how we teach basic extraction theory. Who knows? If you do, email me at firstname.lastname@example.org. I want to learn more about this topic.
: Interestingly, not everyone can immediately identify bitterness from sourness in coffee. I spoke with a Q Grader who told me that they sometimes have to train people to identify the two.
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