I spent four years in a degree (and many years before and after that) learning the rules and norms of classical singing. What was expected and what was acceptable were very important parameters to understand, and depending on the age of the music you were singing, those parameters shifted. For example, the size of your vibrato was expected to be different for early music versus romantic era music. The way you approached grace notes changed from era to era. Trills were different, as was the amount of legato that was considered tasteful. I could give you four thousand more examples.
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