A system for selectively C, Z or half-folding sheet(s) of paper of variable format, comprising: a pocket-like channel (10) with a stop element (12) for defining a paper sheet position and positioning the sheet(s) according to a location of a recipient's address A and a type of fold required; two folding rollers (2, 4) arranged diametrically opposite to each other and forming a folding/nipping area (6); an inducing element (8) movable towards and away from the folding/nipping area for defining a fold line position; and a curved guiding path (14, 16, 18, 20, 22, 24, 26) which substantially covers at least one of the two folding rollers for receiving and guiding the sheet(s) completely around one or the other of the two folding rollers without changing their rotational direction during the folding cycle.