This research focuses on practices and texts related to hand mnemonics in fate prediction methods in contemporary China and Taiwan (with a current focus on Taipei). This skill consists in using the palm of the hand to perform various operations by browsing digital phalanges with the thumb. My project draws on the textual analysis of divinatory manuals as well as on a detailed ethnography of divinatory techniques that are practiced by both specialist diviners and non-specialists. The general aim of the project is to analyze hand mnemonics as a specific pattern of knowledge designed to compute cosmological data in order to deal with an uncertain environment. I examine the mental operations at work in hand mnemonics and focus in particular on division and counting operations that aim to produce remainders. Considered in terms of modular arithmetic principles, these operations can be analyzed as cognitive processes of simplification that attempt to make divinatory information comprehensible to the human mind.