Abstract: A printing unit comprises a pair of blanket cylinders (2) that form a nip through which a substrate can pass as ink images are transferred from the blanket cylinders to both sides of the substrate. Each blanket cylinder (2) is provided with a pair of plate cylinders (1A, 1B) carrying inked lithographic plates which transfer ink images to their respective blanket cylinder. While one plate cylinder (1A, 1B) is in contact with its blanket cylinder (2) the lithographic plates on the other plate cylinder (1A, 1B) can be changed. The other plate cylinder can then be brought into contact with the blanket cylinder (2) and the first plate cylinder removed from contact to allow its plates to be changed. The plate cylinders on one side of the substrate can be moved independently of those on the other side of the substrate. The blanket cylinders (2) can also be moved to release the substrate.