这个问题提法不妥——因为复合句分为两类:并列复合句歌主从复合句。
并列复合句是由一些并列连词(and/but/or…)连接的句子,各自地位平等,没有主从之分。例如: The boy was reading, and his sister was cooking. (两句并列,没有主从之分)
反之,主从复合句(主语从句、宾语从句、表语从句、状语从句、同位语从句)中,从句处于从属地位。比如: When the boy was reading, his sister was cooking.(时间状语从句只起一个说明时间的作用,主句才是句子要强调的)。