The Kitāb al-Fihrist of Ibn al-Nadīm (d. 380/990), one of the most important bio-bibliographical works in Arabic, is an invaluable source for the study of the first four centuries of Arabic writerly culture and of medieval Islamicate history. Ibn al-Nadīm divides his work into 10 parts (and 30 chapters), organised according to different fields of knowledge and scholarship. He also subdivides the notices, entries and titles very deliberately, typically chronologically. Here, I describe another organisational phenomenon discernible in the third chapter of the third part of the Fihrist, namely organization based on notions I term ‘proximity’ and ‘resemblance’, which yield ‘sidebars’ and ‘clusters.’ This organisation yields valuable information about the writerly culture of the 3rd/8th, 4th/9th and 5th/10th centuries.