A carbon atom is classified as primary, secondary or tertiary according to the number of other carbon atoms directly bonded to it: a primary carbon is bonded to one other carbon, a secondary carbon to two, and a tertiary carbon to three. This classification matters because it governs how a carbon and the intermediates formed at it behave, for example the relative stability of the carbenium ions and radicals that form there.

碳原子按与其直接相连的其他碳原子数目分类为伯、仲或叔碳:伯碳与一个其他碳相连,仲碳与两个相连,叔碳与三个相连。这一分类之所以重要,是因为它决定某个碳及在该碳上生成的中间体的行为,例如在该处形成的碳正离子和自由基的相对稳定性。