Abstract 

This article draws on extensive studies of Anton Webern’s Sketchbooks at the Paul Sacher Foundation, Basel. It seeks to demonstrate a cipher-system identified in Webern’s music; to date in Opp. 6, 18 and 31. The cipher-system is explained by way of ‘mapping’ Fortean Pitch Class Set Theory onto the alphabet, whereby each pitch class integer represents a pitch of the chromatic scale whilst simultaneously corresponding to each letter of the alphabet, resulting in what has been coined ‘Alphabet Class Set Theory’.