Basic Guidelines For English Spellings
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(forming nouns) denoting a person or thing performing the action of a verb, or denoting another agent.
From Latin, sometimes via Anglo-Norman French -eour or Old French -eor (see also -ator).
Forming adjectives expressing a comparative sense.
Via Anglo-Norman French from Latin.
Forming nouns denoting a state or condition.
From Latin, sometimes via Old French -or, -ur.