referring now to the figures of the drawing in detail and first , particularly , to fig1 thereof , there is shown a sheet - processing machine , for example a printing press 1 . the assembly includes a feeder 2 , at least one printing unit 3 and / or 4 and a delivery 6 . sheets 7 are removed from a sheet stack 8 and are fed in a separated or overlapping manner to the printing units 3 and 4 via a feed table 9 . the printing units 3 and 4 comprise in a known manner in each case one plate cylinder 11 , 12 and blanket cylinders 15 , 20 which interact in each case with the latter . the plate cylinders 11 and 12 have in each case one clamping apparatus 13 , 14 for fastening flexible printing plates . moreover , each plate cylinder 11 , 12 is assigned an apparatus 16 , 17 for changing printing plates in a semiautomatic or fully automatic manner . the sheet stack 8 rests on a stack board 10 which can be raised in a controlled manner . the sheets 7 are removed from the upper side of the sheet stack 8 by means of a so - called suction head 18 which has , inter alia , a number of lifting and dragging suckers 19 , 21 for separating the sheets 7 . moreover , blowing devices 22 are provided for loosening the upper sheet layers and sensing elements 23 are provided for tracking the stack . a number of lateral and rear stops 24 are provided for aligning the sheet stack 8 , in particular the upper sheets 7 of the sheet stack 8 . the apparatuses 16 , 17 are structurally identical and will therefore be described using primarily by reference to the apparatus 16 . the apparatus 16 has a guide element 27 for a printing plate 28 . the guide element 27 is arranged pivotably on a protective means 29 which covers the cylinder and roll region in front of the printing unit 3 . the guide element 27 is arranged such that it can be pivoted from a receiving position “ a ” for the printing plate 28 , in which it is prealigned by means of provided stops 30 , 31 , into an operating position “ b ” for feeding the printing plate 28 to the plate cylinder 11 . the guide element 27 has a guide plane 32 which , in the operating position “ b ”, forms an extension of a printing plate clamping pad 33 of the clamping device 13 of the plate cylinder 11 and is arranged on the guide element 27 such that it can be pivoted on a lever 37 . furthermore , two transport rollers 34 , 36 are arranged on the guide element 27 , which transport rollers 34 , 36 are jointly in working contact , in order to transport the printing plate 28 or to fix it in the receiving position “ a ”. here , the transport roll 34 which is in contact with the rear side of the printing plate 28 is arranged directly on the guide element 27 , and the transport roll 36 which is in contact with the imaged printing plate front edge is mounted on the pivotably arranged lever 37 . a printing plate flexing roller 41 is mounted rotatably at one end which is remote from the bearing point 38 of the lever 37 . in the operating position “ b ”, the printing plate flexing roller 41 lies between one end of the guide face 32 and the printing plate clamping pad 33 . here , the printing plate flexing roller 41 presses onto a defined location on the printing plate 28 . the defined location corresponds to the bent location 44 which a used printing plate has when it is reused . this bent location 44 is produced by the transition from the clamping device 13 to the circumference of the plate cylinder 11 . in order to feed a new printing plate 28 or one which has already been used to the plate cylinder 11 , the printing plate 28 is first of all prealigned on the stops 30 , 31 in the receiving position “ a ”. the guide element 27 then pivots about its bearing point 42 into the operating position “ b ” for feeding the printing plate . here , the printing plate flexing roller 41 optionally presses onto the bent location 44 of the used printing plate 28 . the printing plate flexing roller 41 is then pivoted in the direction of the plate cylinder 11 until the printing plate front edge is aligned parallel to the printing plate clamping pad 33 . as soon as the printing plate front edge is parallel to the clamping pad 33 , the transport rolls 34 , 36 transport the printing plate 28 into the clamping device 13 . the printing plate is prealigned on the stops 30 , 31 in such a way that the printing plate is positioned obliquely in relation to its front edge by a small angular offset a and is prealigned by a lateral offset s with respect to the register pins 43 , 44 , also referred to as a first register device 43 and a second register device 44 . the printing plate 28 is then fed to the register pins 43 , 44 obliquely and with the lateral offset s . a spacing a between the stop 30 for the prealignment of the printing plate 28 and the register pin 43 of the plate cylinder 11 is smaller than a spacing b between the stop 31 for the prealignment of the printing plate 28 and the register pin 44 of the plate cylinder 11 . at its front edge , the printing plate has two receptacles 46 , 47 which , for prealignment , are first of all brought into bearing contact with the stops 30 , 31 of the printing plate feed apparatus ( in the direction of arrow c in fig4 ) and later with the register pins 43 , 44 on the plate cylinder 11 . the stop 30 and the register pin 43 have a circular cross section and , for example as viewed in the printing plate feed direction , are arranged on the right hand side of the printing plate . the receptacle 46 which can be brought into contact with said aligning means 30 , 43 likewise has a contour which is adapted to the circular shape , run up slopes 48 being provided which are arranged in a funnel shape and facilitate the engagement of the register pin 43 into the receptacle 46 . the stop 31 and the register pin 43 have a square or rectangular cross section and , as viewed in the printing plate feed direction , are correspondingly arranged on the left hand side of the printing plate . the receptacle 47 which can be brought into contact with said aligning means 31 , 44 likewise has a contour which is adapted to the cross sectional shape , with run up slopes 49 which are arranged in a funnel shape . when the printing plate 28 is fed to the plate cylinder 11 , said printing plate 28 is first of all prealigned on the stops 30 , 31 . here , the printing plate 28 assumes an oblique position in relation to its aligned position on the plate cylinder and is arranged to the side by the offset s in the printing plate feed direction , for example to the left . the printing plate is then fed in this position to the register pins 43 , 44 of the plate cylinder 11 . here , the printing plate 28 first of all comes into contact by way of the run up slope 48 of the receptacle 46 with the circular register pin 43 . as a result of the run up slope 48 , the printing plate is displaced laterally to the right , as viewed in the printing plate feed direction , until the register pin 43 engages into the circular receptacle 46 . the printing plate is then pivoted about the register pin 43 until the register pin 44 also engages into the receptacle 47 . the printing plate is then aligned on the plate cylinder with regard to the register pins 43 , 44 and can be clamped by means of the provided clamping elements 13 . after the printing plate 28 has been gripped and clamped fixedly , the lever 37 is pivoted away with the printing plate flexing roller 41 , the guide plane 32 and the transport roller 36 . as a result of this measure , the printing plate 28 can be wound up by rotation of the plate cylinder 11 , without the guide elements being in contact with the imaged side of the printing plate 28 . the printing plate is fed by means of the transport rollers 34 , 36 and the pivoting movements of the guide element 27 and the lever 37 take place by way of actuating means which can be actuated remotely , preferably in an automatically controlled manner .