A cartabla is a handheld praxic device normally forbidden to the avout under the Discipline but widely used Extramuros for navigation. Analogous to a Global Positioning System Device, Fraa Erasmas disappoints Fraa Jad during the early days of their peregrination by purchasing a cartabla to find their way to the Convox insteading of relying on First Principles to build a sextant. The cartabla's failure to display their position (though it works normally in all other respects) is the avout's first sign of the Geometers' interference with Arbre's praxis.