In mathematics, a developable surface is a smooth surface with zero Gaussian curvature. That is, it is a surface that can be flattened onto a plane without distortion. Conversely, it is a surface which can be made by transforming a plane. In three dimensions all developable surfaces are ruled surfaces.

What has an undevelopable surface?

An undevelopable surface is any surface that cannot be flattened into a map without stretching, tearing, or compressing itself, as for example the surface of a sphere.

What do you mean by developable surface?

: a surface that may be imagined flattened out upon a plane without stretching any element.