So there's a particular quirk of English grammar that I've always found quite endearing: the exocentric verb-noun compound agent noun. It appears in a definite, remarkably narrow period - not more than 150, 200 years - before dying out, leaving loads of legacy words in its wake.
To explain briefly! An agent noun is a noun derived from another word (usually a verb), meaning "someone who [verb]s." e.g.: "to travel" => "traveller" "to rule" => "ruler" "to direct" => "director"
In Middle and Modern English, agent nouns derived from verbs are almost always constructed using the agentive suffix -er (from German), less commonly from -or (from French). (Agent nouns derived from nouns usually take -eer or -ist, both from French, but I digress.)
