Another explanation is that the extant Prakrit manuscripts contain scribal errors. Written Sumerian was used as a scribal language until the first century AD. In the Nowell Codex, the lack of scribal regularization is of note. Other activities include the study and practice of heraldry and scribal arts (calligraphy and illumination). Kufic was used primarily in decoration, while Naskh served for everyday scribal use. Also, scribal errors provide evidence of changes in pronunciation over time. Scholars disagree over when scribal culture developed. Ong suggests scribal culture is defined by an alphabet. There were probably scribal schools where members of the aristocracy were taught to write. Latin Extended-D also has characters for medieval scribal abbreviations. These variations may have occurred through scribal error or scribal license. This innovation may have centered on the scribal work of the Stoudion monastery in Constantinople. Bureaucratic organization was diversified, with new branches being formed and scribal duties increasingly specialized. Supporters of the theory dismiss these as scribal mistakes.