Archival research is the systematic location and evaluation of unpublished documents, correspondence, registers, administrative papers and personal papers, held in archives rather than published as books. Its foundational method was set out in 1898 by three Dutch archivists, Samuel Muller, Johan Feith and Robert Fruin, in a manual usually called simply the Dutch Manual, which established the principle of respect des fonds, that a record's meaning depends on the collection it was created and kept within and that archivists must preserve that original order rather than reorganizing documents by subject. The manual gave archival work a professional method distinct from library cataloguing and from history writing itself.
Religious institutions are among the heaviest producers of the kind of unpublished record archival research specializes in, parish registers, monastic account books, mission correspondence, diocesan visitation reports and internal denominational minutes, almost none of which was written for publication or for a general reader. Where a tradition's own published history smooths over a controversy or omits a schism, the archival record of the institution's internal correspondence is frequently where the disagreement survives in the participants' own words, which is why the atlas treats archival findings as a distinct and often corrective category of evidence.