 The smallest member of the family, consisting of four fused cyclopropane rings, is [3.3.3.3]fenestrane, which has systematic name tetracyclo[2.1.0.01,3.02,5]pentane and is also called pyramidane.The next symmetric member, [4.4.4.4]fenestrane, has four cyclobutane rings fused, and has systematic name tetracyclo[3.3.1.03,9.07,9]nonane.The rings need not all be the same size as each other, so [4.4.4.5]fenestrane has three cyclobutane rings and one cyclopentane ring.