If you have a bass or guitar riff then you would want to do a harmonic analysis to determine the key.
I am not studied in music theroy. However isnt the first note you play what determins the key?
Looking back on my previous comment. If you have a riff that you made and you are trying to analyze it. The root note could also be the note that is used the most.
Usually that is true, but a big part of the progressive genre is subversion of expectations. It could be a good creative decision to create tension by not clearly establishing a key center then have that part lead into a very tonality stable part with a lot of rhythm complexity.
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