Middle American archaeological research has always been especially centered in the Maya region of Guatemala and Yucatan, for here, apparently, ancient American civilizations reached their culmination in art, architecture, writing, and probably in mathematics and astronomy. There are more specialists who limit their researches to one field in the Maya area than in any other branch of American archaeology, and among these are many whose sole interest is in the topics of epigraphy and the calendar. This field is highly specialized and requires experts, but the great value of their results is not per se, but lies in the accurate dating which can be given to developmental phases of architecture, ceramics, and similar products; once the temporal relationships of these are established, they may be used as a time-scale for adjacent regions to which these objects and traits were carried by influence and trade.