A correlation dating technique that can yield a relative or absolute age or chronology. The basis of cross-dating is the occurrence of finds in association. The assumption is that a particular type of artifact, for example a type of sword, when found in an undated context will bear a similar date to one found in a dated context, thus enabling the whole of the undated context to be given a chronological value. The method is based on the assumption that typologies evolved at the same rate and in the same way over a wide area or alternatively on assumptions of diffusion. Many of the chronologies constructed before the advent of chronometric dating techniques were based on cross-dating. New techniques such as radiocarbon dating showed some of the links established by cross-dating to be invalid, so the method has become somewhat discredited. However, its use is still helpful where recognizable products of dateable manufacture are found in undated contexts with no possibility of using a chronometric dating technique. So in the absence of geochronology, two cultural groups can only be proved contemporary by the discovery of links between them. If in culture A an object produced by culture B is found, A must be contemporary with, or later than, B. The term cross-dating ought strictly to be used only when an object of culture A is also found in proved association with culture B, when overlap of at least part of the time span of each is proved. Items having an established date, such as dated coins or buildings, or ceramics of known manufacture are most often used. By itself, a cross-dated chronology does not give absolute dates, but it may be calibrated by reference to other dating methods. A type of cross-dating has always been used in geology and stratigraphical sequences are often correlated by the assemblages of fossils they contain; this is known as biostratigraphy. The archaeological versions of cross-dating may have been developed directly out of the geological method and may have been based on a false analogy between biological fossils and archaeological artifacts.