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Those aren't slashers. Horror films, yes. I keep telling you, Friday the 13th is mainly responsible for the slasher as we know it today.
Bay of Blood - 1971 (Italian giallo)
Black Christmas - 1974 (Canadian suspense thriller)
The Town that Dreaded Sundown - 1976 ('based on true story')
Schizo - 1976 (not a slasher)
Halloween - 1978 (suspense thriller influenced by Italian films and Hitchcock)
Tourist Trap - 1979 (Texas Chainsaw Massacre rip off)
Yeah they ripped off a few kills, doesn't matter though. The plot is derivative from Halloween, the writer even admits this.And if you've never seen Bay of Blood, you should watch it as some of the kills and setups from many slashers are directly ripped off (including the Kevin Bacon kill from F13)
Giallo predates giallo films. Giallo means 'yellow' in Italian. Yellow being the colour of the books they were based off.Yes many slashers are influenced by various gialli and other murder/mystery films, but saying that Halloween isn't a slasher is like saying that The Girl Who Knew too Much isn't a giallo because it was so early (maybe the first) in the SUB-genre, or that Nosferatu (1922) isn't a vampire film because it was before Horror of Dracula (1958).
Vampire mythology obviously predates vampire films.
Slashers are a movie genre, spawned by the popularity of Halloween and other films.