Words of Disburdenment
The contents of the Patimokkha (except for the Nidana) are found almost word-for-word in the Suttavibhanga, the first part of the Vinaya Pitaka (first part of the major religious text of Theravada Buddhism). The first version of these texts was as a discourse read aloud at a meeting of bhikkhus (Buddhist monks) held twice a month, during which any bhikkhu who had committed any of the offenses would come forward, admit the offense and, if need be, begin the punishment.
"[E]very single Rule or Clause in the Pâtimokkha is in fact [f]ound word for word in the Sutta-vibhanga, the quotations being so complete that the Pâtimokkha might be entirely put together again by piecing together extracts from the Vinaya Pitaka. And it is not possible that the Pâtimokkha originated merely by such a process of dovetailing; for the quotations in the Vinaya Pitaka, though not actually called quotations, bear the unmistakable stamp of being taken from some pre-existing work. The cause which led to the Pâtimokkha, and the Upasampadâ-kammavâka, being separately preserved at all, is the same as the cause which led to their exclusion from the lists of the Pitaka textsthe fact, that is, of their being liturgical compositions."1
The Patimokkha also contains a set of rules relating specifically to bhikkhunis (Buddhist nuns), as the contents of the Suttavibhanga deal mainly with bhikkhus (although the gender of the student does not matter for many rules). Of the 311 rules in the Bhikkhuni Patimokkha, 181 are adapted from the Bhikkhu Patimokkha, 33 are identical to rules in the Khandhakas or the Bhikkhu Patimokkha, and 8 are elaborations of a rule in the Pakittiya (of the Bhikkhus Patimokkha). The remaining 87 rules are unique, and are sometimes considered to be a sign that bhikkhunis are somehow inferior to bhikkhus. However, more than a third of these unique rules serve to prevent bhikkhunis from becoming victims of "abusive or careless behavior of other bhikkhunis,"2 two of the rules (Pakittiyas 6, 44) serve to prevent bhikkhunis from becoming servile to bhikkhus or lay people, and all but three of the remaining rules (Pakittiyas 59, 94, 95) were created in response to complaints from bhikkhunis about another bhikkhuni's behavior.
Information/interpretation from personal experience and (1:) "Introduction to the Vinaya Texts from the Pâli," available at http://www.sacred-texts.com/bud/sbe13/sbe1302.htm and (2:) Thanissaro Bhikkhu's translation of "the Bhikkhuni Patimokkha," available at http://www.accesstoinsight.org/canon/vinaya/bhikkhuni-pati.html. Text in "double quotes" is a quotation from that text; text in [brackets] within double quotes has been added by me to clarify a point or to correct a typographical error.