fig1 is a perspective depiction of a card according to the invention ; the overall card is designated as 1 . the card has a cover 2 and a back 3 . the cover 2 and the back 3 meet at a fold line f 1 and can be folded against one another along this line . fig1 shows a position in which the cover is folded against the back at a 90 ° angle . inside , between the cover and the back , is a folding system 4 that assumes a spatial , three - dimensional form when the cover 2 is unfolded with respect to the back 3 , as can be seen in fig1 . in the embodiment depicted , the folding system consists of three superimposed frame elements 5 , 6 and 7 . in the frame element that is the greatest distance from the back 3 ( a1 + a2 + a3 ), a window 5a is cut out . in the frame element 6 that is next to frame element 5 in the direction of the back 3 , a window 6a is similarly cut out , but its dimensions are smaller than those of window 5a . between the frame element 6 and the back 3 , there is another frame element 7 , at a distance a1 from the inside surface 3a of the back 3 , which similarly has a window 7a that is even smaller than both of the windows 5a and 6a . fig2 shows a view in the direction of arrow ii of the card unfolded by 90 ° according to fig3 . there it can be seen that the superimposed windows 5a , 6a and 7a , which are formed by the frame elements 5 , 6 and 7 and are successively smaller in that order , are positioned symmetrically to an axis of symmetry s . the individual windows are dimensioned such that the respectively corresponding corners each lie on the imaginary connecting lines v 1 through v 4 . in fig2 only the upper left - hand corners 9 , 10 and 11 are drawn in , which lie on the common connecting line v 2 . since -- as can be seen in fig1 -- the frame elements 5 , 6 and 7 are positioned at spatial intervals with respect to one another and on the other hand the windows are successively smaller as one moves towards the back 3 , the result is an effective spatial , optical effect . the connecting lines v 1 through v 4 meet at corresponding end points e 1 through e 4 on the inside surface 3a of the back 3 ; these end points define an imaginary rectangle 8 . when the cover 2 is swung towards the back 3 , the windows also shift towards one another , although the connecting lines v 1 through v 4 join at the end points e 1 through e 4 in every position , so that this rectangle 8 defined by these end points e 1 through e 4 remains in place regardless of the swinging position . in this way , the vision of the viewer is always directed towards the rectangle 8 . advertising slogans or even names can be imprinted there , for example . in fig4 the structure of the folding system is shown in greater detail . there is can be seen that the folding system has a support frame 12 that assumes a cuboidal form in the 90 ° position of the cover with respect to the back , whereby the frontal edges of the cuboid are open . the support frame 12 has an attachment tab 17 , a bottom side 15 , a side wall 13 , a top side 16 , which forms the upper frame element 5 , and another side wall 14 . the support frame is attached to the inside surface 2a of the cover 2 with the outside surface of the side wall 14 . similarly , the outside surface of the bottom side 15 is attached to the inside surface 3a of the back 3 with adhesive . inside the support frame 12 , the two frame elements 6 and 7 are positioned at the intervals indicated in fig1 . the frame element 6 has laterally bent attachment tabs 21 and 22 that are attached by fold lines f 6 and f 7 to the horizontally positioned frames adjacent to the window 6a . the outside surfaces of the attachment tabs are glued to the inside surfaces of the side wails 13 and 14 . the frame element 7 is correspondingly configured , the only difference being that the window 7a ( not shown in fig4 ) is smaller than the window 6a of the frame element 6 . since the frame elements 6 and 7 are each held in the support frame 12 by fold lines and the support frame itself has fold lines f 2 through f 5 , the support frame folds up when the cover is folded together in the direction of the back , thereby there is a parallel shift in the individual frame elements , until the point at which these elements are essentially superimposed , with no distance between them . fig5 shows an interim stage passed through by the support frame when the card is folded together ( and naturally when it is unfolded as well ). because of this construction and the use of the support frame and of the frame elements positioned inside the support frame , the card can be folded together so as to be very flat , and can also be produced very simply . the individual parts of the card consist of thin cardboard , such as cardboard of a strength of 0 . 2 to 0 . 5 mm . the pressed parts needed are extremely simple , as fig4 and 6 show . the layout of the cover and back need not be shown . it consists of an oblong strip , the middle of which is divided by a fold line f 1 . the layout 19 of the support frame 12 is shown in fig4 . the way in which the attachment tab 17 connects to the bottom side 15 by a fold line f 2 can be seen here . the side wall 13 is then attached to this by the fold line f 3 . this is followed by the top side 16 by the fold line f 4 , which forms the frame element 5 and has the punched out area 5a , which forms the window . the side wall 14 is then connected to this by the fold line f 5 . the figure shows upwardly and downwardly protruding reinforcement tabs 18 , which are intended to reinforce the free bridges that border the window 5a in fig4 above and below , in that they can be folded inwardly . in this area , the frame is twice as strong after these reinforcement tabs have been folded in . the total length of the bottom side 15 and of the side wall 13 comes to l 1 + l 2 , as does the total length of the upper side 16 and the side wall 14 ( cf . fig4 ), and corresponds to the length l 3 of the back 3 or of the cover 2 of the card . because of this , the support frame ends up with the fold edge f 4 on a level with the cover 2 and the back 3 when in the folded together position in this way , the space available inside is optimally utilized . the layout of the two frame elements 6 and 7 is also extremely simple , as fig6 shows . because the frame element 7 is configured in keeping with the layout of the frame element 6 ( except for the size of the window 6a ), only the layout 20 of the frame element 6 is depicted . one can see the two attachment tabs 21 and 22 , as well as the reinforcement tabs 23 on the free side , which can be folded in in order to reinforce the frame in this area . the fold lines f 6 and f 7 make it possible to fold and unfold the card ( cf . fig5 ). fig7 shows another embodiment of a folding card . in this embodiment , the two parts 26 and 27 of the cover 25 are connected to the rectangular back 3 along the longitudinal edges 3d and 3c by fold lines . there is a tab 29 on the free edge of part 27 of the cover . a corresponding slit recession 28 is configured in the part 26 of the cover , so that whenever the only schematically depicted folding system 4 is folded together towards the back 3 in the direction of the arrow p , the card can be closed by inserting the tab 29 into the slit . fig8 shows the card with the basic design explained on the basis of fig7 in an open position in the individual windows , design elements 30 , 31 and 32 are shown , which when viewed in the direction of arrow a combine into a complete spatial design . for example , the overall image of a city can be depicted on the back wall 3 . the individual design elements 30 , 31 and 32 then repeat individual designs from the overall image and in this way emphasize these individual designs from the overall image on the back . the design elements can be stamped out during the stamping of the frame elements . of course , it is also possible to affix corresponding design elements 30 through 32 on the windows from the side that cannot be seen . it is clear that the invention can naturally be realized with more than three superimposed frame elements . the more frame elements that are used , the more detailed will be the spatial impressions generated . however , this is offset by the somewhat greater expenses involved in producing and assembling the card . the windows can be nearly any shape , such as round or rectangular . in any case , the elements used can be appropriately imprinted prior to assembly , so that there are spatial images when the card is unfolded .