The four-membered cyclic C4H2 resulting from electrocyclization of butadiyne has been found 75.2 kcal/mol above the latter species. In its ground state, it is a singlet non-planar biscarbene of C2 symmetry. The electronic structures of the singlet and triplet have been studied using ELF analysis. The opening into butadiyne needs ca. 20 kcal/mol activation and follows a pathway which does not preserve the C2 symmetry. Breaking the carbene-carbene bond results in a bicyclo-biscarbene, a degenerate isomerization which offers a reaction path for the scrambling of C2C3 atoms in butadiyne. Breaking the HC–C bond results in a sigmatropic shift yielding cyclopropylidene.