Directly after a preposition, "who" would still be wrong. I think this is about the only case "whom" is still used regularly. e.g. - There were three people injured, one of whom died later.

*Whom* is used here because it's a pronoun referring to people. You could use *these people* instead with no change of actual meaning.

Answer is as 'Gray' says, the easiest way to know is to try to put the words 'him' or 'he' into the sentence. "Famous nappers include Winston Churchill, John F. Kennedy, Napoleon, Albert Einstein, and Thomas Edison, all of *who* are known to have valued their afternoon naps". = incorrect but used often in current English because many people think whom is archaic and should be abandoned. You cannot replace the word 'who' in this sentence with 'he' or 'him' it does not make sense, so this is probably one of the last few remaining times where you can or should use "whom". even if it is considered archaic by many 1. Winston Churchill who was a famous napper. 2 John F Kennedy who was a famous napper. 3 Napoleon who was a famous napper. 4 Albert Einstein who was a famous napper. 5 Thomas Edison who was a famous napper. Can all be replaced with "he" 1. Winston Churchill HE was a famous napper. 2 John F Kennedy HE was a famous napper. 3 Napoleon HE was a famous napper. 4 Albert Einstein HE was a famous napper. 5 Thomas Edison HE was a famous napper. Ok to use 'who' when speaking of them individually plural has nothing to do with it, people will say 'who' in that sentence and it will only get debated in an English class but accepted by modern speakers in most cases.

Please ignore Belisarius' answers. You can certainly use "who" to refer to plural people. The reason for using "whom" here is because this word is not the subject. The subject is "all." The following phrases use the same structure: "all of them" (NOT "all of they") "all of us" (NOT "all of we") You can't say "all of who," because then "all" and "who" are both acting as subjects. If you remove the word "all," or move it so that it is not the subject of the clause, then you can use the word "who" (because "who" will then be the subject). In that case, you would write: "Famous nappers include [...], WHO are all known to have valued their afternoon naps."

Here *whom* indicates plural number. You can't say *who* referring to several people.