A negative extensibility material structure pulls back and contracts when the external tensile load reaches a certain critical level. In this paper, we reveal basic mathematical features of the nonlinear strain energy function responsible for this unusual mechanical property. A systematic discussion leads to a comprehensive phase diagram in terms of design parameters for a simple unit cell structure that provides a panoramic view of all possible nonlinear mechanical behaviors. A negative extensibility region clearly is identified in the diagram. The sought property is seen to be rare, occurring only for a very narrow range of the design parameters. Nonetheless, due to the simplicity of the studied structure we suggest that the negative extensibility should be a more common phenomenon than previously thought. It can appear in simple bistable cells made of only several linearly elastic links, although at some peculiar combinations of their properties. These bistable unit cells can be used to design periodic mechanical metamaterials whose examples are shown as well as innovative architectural metastructures.