According to this classification, the great majority of the glyphs whose meanings have been determined fall into the first group, and those whose meanings are still unknown into the second. This is particularly true of the inscriptions, in which the known glyphs practically all belong to the first group. In the codices, on the other hand, some little progress has made been in reading glyphs of the second group. The name-glyphs of the principal gods, the signs for the cardinal points and associated colors, and perhaps a very few others may be mentioned in this connection.[18] Of the unknown glyphs in both the inscriptions and the codices, a part at least have to do with numerical calculations of some kind, a fact which relegates such glyphs to the first group. The author believes that as the reading of the Maya glyphs progresses, more and more characters will be assigned to the first group and fewer and fewer to the second. In the end, however, there will be left what we may perhaps call a "textual residue," that is, those glyphs which explain the nature of the events that are to be associated with the corresponding chronological parts. It is here, if anywhere, that fragments of Maya history will be found recorded, and precisely here is the richest field for future research, since the successful interpretation of this "textual residue" will alone disclose the true meaning of the Maya writings. Three principal theories have been advanced for the interpretation of Maya writing: 1. That the glyphs are phonetic, each representing some sound, and entirely dissociated from the representation of any thought or idea. 2. That the glyphs are ideographic, each representing in itself some complete thought or idea. 3. That the glyphs are both phonetic and ideographic, that is, a combination of 1 and 2.