The original order of the Regla Papers is unknown. Many documents—some related, some unrelated—were bound together as cuadernos, while a number of unbound documents appeared to be in no particular order, and some were simply grouped together much as in modern file folders. No overall, systematic arrangement of these many cuadernos and documents appears ever to have existed, or if it did, it has been lost. This lack of order was observed by J. Horace Nunemaker and his associates in the late 1940s when they undertook to simply arrange unbound documents in chronological order. Beyond that effort, no other attempt was made to devise or impose an overall arrangement over the years, nor has an attempt been made to do so in terms of this guide.

In their present arrangement the papers are grouped into five parts, or sequences reflecting something of the provenance, albeit often a provenance of convenience, of each part, as well as the nature of the material, and the format it assumes. The first is the "Gaines Sequence," the part of the papers described in Gaines's Three Centuries of Mexican Documents: A Partial Calendar of the Regla Papers (1963). The second sequence is called the "Gastine-Fieler Sequence." This portion of the papers consists of those documents acquired by the Washington State University Libraries in the late 1960s through the efforts of Mr. Morton Fieler of Chicago, Illinois, from an art dealer in Los Angeles, California. Included in this part are also several cuadernos that the libraries apparently acquired from other sources, and which were added to the Regla Papers in undocumented fashion sometime prior to 1970.

The third part is the "Jesuit Haciendas Sequence." Its provenance traces back to J. Horace Nunemaker's acquisitions in the 1940s, but the documents, having been bound together, were not described in Gaines's Calendar. A short, fourth sequence consists of maps, charts, proclamations, and genealogical documents— disparate items that appear to have possibly been part of the Gastine-Fieler acquisition. The fifth and last part, the "Microfilm Sequence," is microfilm copy, acquired in the mid-1940s, of selective related documents the originals of which the Mexican authorities apparently refused to allow to leave the country.

This Guide is not a calendar. Rather, it provides brief, descriptive entries for each cuaderno or bundle of documents, as well as for many individual important or bulky documents that are not included in any aggregate. The "Gaines Sequence" entries repeat those of Gaines in her Calendar, but the descriptions have been condensed.

Each entry is numbered and contains a Spanish title, usually taken from the original document. Generic titles have been supplied for those documents lacking distinctive titles. The titles have been transcribed using standard Spanish, with spellings made uniform and interpolation brought to bear on abbreviations and certain conventions in the original to help clarify texts and dates when necessary. The entries also include the date(s) of the document(s) whenever possible, and the number of leaves comprising a document, although in one instance, Section III, the "Jesuit Haciendas Sequence," Tomo 1 (pp. 42-67), the total number of leaves is indicated, not the number of leaves of the individual documents. The final element of each entry is an English-language abstract or description of the document in terms of content.

The index, while not exhaustive, is designed to facilitate access to the entire collection. Essentially patro-toponymic in substance, it does, however, contain references to some of the more important subjects found in the collection as well. The references, as should be obvious, are to the numbered entries in the guide, not to pages. Those index items bearing the prefix "mcf" refer to the documents in the "Microfilm Sequence."