The use of variable-period grid steps makes it possible to reduce the distortion of results caused by the unequal availability of securely-dated charters in the Corpus over time. Very short or very long ones are relatively uncommon (fig. The charter text is stored as a part of the textual structure called Source Document, which is a plain text document created using Extensible Markup Language (XML).When variable periods are being applied, all dates are initially mapped to the three-year period step and then some of the dates are remapped to wider periods to reduce fluctuations in the number of charters available for any given period over time (fig. A Source Document can easily be parsed by a computer or read by a person.
Obviously, the number of charters available for each chronological time span varies (fig.
1), as does the accuracy of the chronological evidence which varies from the exact day, month and year to a range of several years.
The capacity to establish chronological boundaries for the individual medieval charter is particularly important in the case of England where, from the Norman Conquest in 1066 until the beginning of the reign of Richard I in 1189, only the occasional document issuing from the royal chancery bore a date.
Over one million private charters survive from the twelfth and thirteenth centuries, but no more than eight percent of them can be accurately dated.
The set of periods can be viewed as a chronological grid that covers a given time span (in our case from the year 1050 to 1359). Number of charters in the DEEDS Corpus over time Grid steps can be constant (for example 3, 5, 10 or more years) or variable, to reflect the availability of charters in the given time span.