The film’s title is set in Gemini, a typeface shown in Photo-Lettering’s Alphabet Thesaurus Vol. 3 and also Photo Lettering’s One Line Manual of Styles, both published in 1971. The latter volume shows four weights, labelled Gemini 4 through 7. The weight used on the poster looks to be Gemini 5 or 6. This typeface choice may have been directly influenced by a 1972 paperback edition of Blatty’s novel published by Bantam Books, which also uses Gemini.
Additional text on the poster is either set in or based upon Kabel.