A keynote is that symptom which is one that sets it apart from most other remedies. It's the one where if some other remedy is playing in the key of B-flat minor and your remedy is in C major, it should sound out of tune. Kent's artistic method is alluded to here. The keynote of the remedy is understood much better after study of much materia medica. Then intuition can guide effectively to a remedy.
Sir John Weir said that we should take the minimum number of symptoms with the maximum value. Grabbing just the keynotes of the remedy from the patient is the ideal.
Study Clarke's grand characteristics, Kent's particulars, Allen's, Lippe's and Guersey's keynotes, Schmidt's cardinal or pilot symptoms, or Nash's leaders. They all mean the same thing.