Although it's one of his most successful films, Le chat à neuf queues (1971) (Il gatto a nove code) is the least favorite of Dario Argento among his pictures.
The scene where Giordani meets Anna Terzi at her father's house is a reference to the scene in the novel "The Big Sleep" by Raymond Chandler where Marlowe first meets Vivian Regan at her father's house. Portions of the dialogue are verbatim.
During the nighttime burglary at the Terzi institute a doctor can be clearly heard whistling Ennio Morricone's tune from Dario Argento's movie L'oiseau au plumage de cristal (1970) opening.
The title does not refer to a feline or a kind of whip, but the number of leads to follow in the attempt to solve the murders.
On one gravestone at the cemetery is written "Di Dario" = "Dario's."