Back cover copy
The book's back and its blurb: a few lines on which a decisive share of the purchase depends.
The back cover (French: quatrième de couverture, "fourth cover") carries the book's blurb: the few lines a reader scans in the bookshop between the title that drew them and the buying decision.
Its grammar differs radically from the synopsis: the blurb teases without resolving — setting the situation and the promise, never the ending. Its recurring ingredients: a hook, the protagonist and their world, the tipping element, an open question, and positioning (genre, tone). Writing it is an exercise in literary copywriting many authors find harder than expected: condensing 400 pages into 120 words that create hunger.
Example
"She thought she had buried her past. Someone just dug it up — and mailed her a photograph."