杜佑
Being a relatively quickly compiled work of official history, the "Old Book of Tang" was a compilation of earlier annals, now lost; it further incorporates other monographs and biographies, using as sources (for instance) the "Tongdian" of Du You. These sources were often directly copied from records and earlier histories, and the result would be severely criticised during the Northern Song; Emperor Renzong of Song, for example, called the book "poorly organised, burdened with unimportant details, wanting in style and poorly researched". These errors even included duplicated biographies of characters.