A form element for forming at least one part of a formwork for a concrete part, - wherein the form element in its first state is flexible, three-dimensionally deformable and can be formed into a double-curved, in particular non-unwindable, surface, - wherein, in its second state, the form element is stiffened and solidified, in particular in a deformed position of the form element, - and wherein the form element comprises at least two flexible layers, which are arranged one above the other in a planar manner, - wherein in the second state, the layers are connected to one another, in particular reversibly, in order to solidify the form element, - wherein, in the second state, the layers are pressed together in a planar manner and are thereby non-positively connected to one another, wherein in the second state, at least one layer is connected to at least one other layer, in particular to an adjacent layer, transversely, in particular normal, to its surface extension in order to solidify the form element, characterised - in that, in the first state, at least one layer is arranged so as to be movable along its surface extension relative to at least one other layer, in particular relative to an adjacent layer, - and in that the form element can be brought into its second state both in an undeformed position and in a three-dimensional position deformed to a double-curved surface of the first state for its stiffening and solidification. - wherein the form element in its first state is flexible, three-dimensionally deformable and can be formed into a double-curved, in particular non-unwindable, surface, - wherein, in its second state, the form element is stiffened and solidified, in particular in a deformed position of the form element, - and wherein the form element comprises at least two flexible layers, which are arranged one above the other in a planar manner, - wherein in the second state, the layers are connected to one another, in particular reversibly, in order to solidify the form element, - wherein, in the second state, the layers are pressed together in a planar manner and are thereby non-positively connected to one another, in the second state, at least one layer is connected to at least one other layer, in particular to an adjacent layer, transversely, in particular normal, to its surface extension in order to solidify the form element, characterised - in that, in the first state, at least one layer is arranged so as to be movable along its surface extension relative to at least one other layer, in particular relative to an adjacent layer, - and in that the form element can be brought into its second state both in an undeformed position and in a three-dimensional position deformed to a double-curved surface of the first state for its stiffening and solidification. - in that, in the first state, at least one layer is arranged so as to be movable along its surface extension relative to at least one other layer, in particular relative to an adjacent layer, - and in that the form element can be brought into its second state both in an undeformed position and in a three-dimensional position deformed to a double-curved surface of the first state for its stiffening and solidification.