Addendums are additional documents or pieces of information that are added to an existing agreement, contract, or report. Synonyms for addendums include annex, appendix, attachment, rider, schedule, codicil, supplement, and postscript. An annex is an attachment that is added to a document to provide more details. Appendix refers to extra information that is added to a book or document. An attachment is a supplemental document that is added to the original document. A rider is a clause that is added to a document to modify its original terms. A schedule is a list of detailed or additional information that is included in a document. Codicil is a legal document that amends an existing will. Lastly, a postscript is an additional remark or note that is added at the end of a letter or document.