An apparatus for folding printed sheets, comprising • a guide plane, in which a printed sheet (1, 1', 1") can be made available in a folding position (33, 33', 33", 36', 36"), • at least two folding rollers disposed on a first side of the guide plane, each having a rotational axis and between them forming a folding gap for the printed sheet (1, 1', 1") that is to be folded, the rotational axes of said folding rollers being oriented parallel to one another as well as substantially parallel to the guide plane, • a compressed-air device, oriented substantially parallel to the rotational axes of the folding rollers and located on a second side lying opposite the first side of the guide plane in the region of the folding gap, said compressed-air device being connected to a compressed-air source and to a control unit and having at least one outlet (11, 11', 11 ") for compressed air, which is directed at the folding gap, characterized in that the apparatus (2, 2', 2", 2"') includes control elements (17, 18, 20; 17', 17", 20'), connected to the compressed-air source and to the control unit, for varying the duration of the supply of compressed air to the at least one outlet (11, 11', 11") of the compressed-air device and/or for varying the cross-sectional area (19, 19', 19") of said outlet (11, 11', 11") and/or for varying the pressure of the compressed air delivered to said outlet (11, 11', 11"), in which case the control elements (17, 18, 20; 17', 17", 20') can be controlled via the control unit in such a way that the duration of the supply of compressed air to the at least one outlet (11, 11', 11") and/or the cross-sectional area (19, 19', 19") of said outlet (11, 11', 11") and/or the pressure of the compressed air delivered to said outlet (11, 11', 11") can be adapted to the properties of the available printed sheet (1, 1', 1 "). • a guide plane, in which a printed sheet (1, 1', 1") can be made available in a folding position (33, 33', 33", 36', 36"), • at least two folding rollers disposed on a first side of the guide plane, each having a rotational axis and between them forming a folding gap for the printed sheet (1, 1', 1") that is to be folded, the rotational axes of said folding rollers being oriented parallel to one another as well as substantially parallel to the guide plane, • a compressed-air device, oriented substantially parallel to the rotational axes of the folding rollers and located on a second side lying opposite the first side of the guide plane in the region of the folding gap, said compressed-air device being connected to a compressed-air source and to a control unit and having at least one outlet (11, 11', 11 ") for compressed air, which is directed at the folding gap, characterized in that the apparatus (2, 2', 2", 2"') includes control elements (17, 18, 20; 17', 17", 20'), connected to the compressed-air source and to the control unit, for varying the duration of the supply of compressed air to the at least one outlet (11, 11', 11") of the compressed-air device and/or for varying the cross-sectional area (19, 19', 19") of said outlet (11, 11', 11") and/or for varying the pressure of the compressed air delivered to said outlet (11, 11', 11"), in which case the control elements (17, 18, 20; 17', 17", 20') can be controlled via the control unit in such a way that the duration of the supply of compressed air to the at least one outlet (11, 11', 11") and/or the cross-sectional area (19, 19', 19") of said outlet (11, 11', 11") and/or the pressure of the compressed air delivered to said outlet (11, 11', 11") can be adapted to the properties of the available printed sheet (1, 1', 1 ").