For large base-sizes, some of the cyclobutanes can be fused anti to each other, giving a non-convex polygon base. These are geometric isomers of the prismanes. Two isomers of [12]prismane that have been studied computationally are named helvetane and israelane, based on the star-like shapes of the rings that form their bases. This was explored computationally after originally being proposed as an April fools joke. Their names refer to the shapes found on the flags of Switzerland and Israel, respectively.