Compounds having one atom (usually a quaternary carbon) as the only common member of two rings, e.g.
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Source:
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1366 (https://doi.org/10.1351/pac199567081307)