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. <EOS>