|The Journal of Studies in Langauge 28.2, 199-217. It is traditionally prescribed in English grammar that object relative pronouns are optionally omitted while subject relative pronouns are not. This research addresses the issue: why the two relative pronouns behave differently. For this purpose, this research employs the key concept of clausal boundary marking. Subject and objects occupy different positions in English: subjects are positioned before verbs and objects are after verbs. This paper shows that the fixed word order in English has to do with a different degree in the omission of relative pronouns. What is more, it is suggested in this research that the key factor is not whether relative pronouns are a subject or an object. This paper demonstrates that relative pronoun omission depends on whether the clausal boundary is maintained or not, even after relative pronoun omission.