The radical stability order describes the trend in the stability, and hence the ease of formation, of alkyl radicals: tertiary is more stable than secondary, which is more stable than primary, which is more stable than the methyl radical. It matters because a more stable radical forms more readily, so this order predicts where halogenation occurs. Radicals that are resonance-delocalised, such as allylic and benzylic radicals, are stabilised still further.

自由基稳定性顺序描述烷基自由基稳定性(从而生成难易)的规律:叔自由基比仲自由基稳定,仲自由基比伯自由基稳定,伯自由基又比甲基自由基稳定。它之所以重要,是因为越稳定的自由基越容易生成,故这一顺序可预测卤代反应发生的位置。共振离域的自由基,如烯丙基和苄基自由基,则更为稳定。