the same text; thus, the Egyptian historian Manetho's history of Egypt is only known by extensive references to it made by subsequent writers, such as Eusebius and Sextus Julius Africanus, and the dates for the same pharaoh often vary substantially depending on the intermediate source. Regnal periods have to be pieced together from inscriptions, which will often give a date in the form of the regnal year of the ruling pharaoh. Yet this only provides a minimum length of that reign and may or may not include any coregencies with a predecessor or successor. In addition, some Egyptian dynasties probably overlapped, with different pharaohs ruling in different regions at the same time, rather than serially. Not knowing whether monarchies were simultaneous or sequential results in widely differing chronological interpretations. Where the total number of regnal years for a given ruler is not known, Egyptologists have identified two indicators to deduce that total number: for the Old Kingdom, the number of cattle censuses; and for later periods, the celebration of a Sed festival. A number of Old Kingdom inscriptions allude to a periodic census of cattle, which experts at first believed took place every second year; thus records of as many as 24 cattle censuses indicate Sneferu had reigned 48 years. However, further research has shown that these censuses were sometimes taken in consecutive years, or after two or more years had passed. The Sed festival was usually celebrated on the thirtieth anniversary of a pharaoh's ascension, and thus rulers who recorded celebrating one could be assumed to have ruled at least 30 years. However, once again, this may not have been standard practice in all cases. In the early days of Egyptology, the compilation of regnal periods was also hampered by a profound biblical bias on the part of Egyptologists. This was most pervasive before the mid 19th century, when Manetho's figures were recognized as conflicting with biblical chronology, based on Old Testament references to Egypt (see Pharaohs in the Bible). In the 20th century, such biblical bias has mostly been confined to alternative chronologies outside the scholarly mainstream. Synchronisms A useful way to work around these gaps in knowledge is to find chronological synchronisms, which can lead to a precise date. Over the past decades, a number of these have been found, although they are of varying degrees of usefulness and reliability. Seriation, i.e. archeological sequences. This does not