Documents may be grouped according to their date, according to their place of origin, according to their contents, according to their form.[97] Here we have the four categories of time, place, species, and form; by superposing, then, we obtain divisions of smaller extent. We may undertake, for example, to make a group of all the documents having a given form, of a given country, and lying between two given dates (French royal charters of the reign of Philip Augustus); or of all the documents of a given form (Latin inscriptions); or of a given species (Latin hymns); of a given epoch (antiquity, the middle ages). We may recall, by way of illustration, the existence of a _Corpus Inscriptionum Græcarum_, of a _Corpus Inscriptionum Latinarum_, of a _Corpus Scriptorum Ecclesiasticorum Latinorum_, the _Regesta Imperii_ of J. F. Böhmer and his continuators, the _Regesta Pontificum Romanorum_ of P. Jaffé and A. Potthast. Whatever the division chosen, there are two alternatives: either the documents to be placed in this division are dated or they are not. If they are dated, as is the case, for example, with the charters issued from the chancery of a prince, care will have been taken to place at the head of each slip the date (expressed in modern reckoning) of the document entered upon it. Nothing is then easier than to group in chronological order all the slips, that is, all the documents, which have been collected. The rule is to use chronological classification whenever possible. There is only one difficulty, and that is of a practical order. Even in the most favourable circumstances some of the documents will have accidentally lost their dates; these dates the compiler is bound to restore, or at least to attempt to restore; long and patient research is necessary for the purpose.