Theory of deformation twinning by Bilby and Crocker is applied to calculate the twinning elements for all possible twinning modes in monoclinic Er2Si2O7 TYPE C and TYPE D. The magnitude of shear strain was also calculated for each twinning mode. The criteria of small shear strain and minimum shuffling is applied to predict the operative twinning mode for monoclinic Er2Si2O7 TYPE C and TYPE D. These predications of the theory are compared with the available experimental information and may act as guideline for future experimental work. (c) 2005 Springer Science + Business Media, Inc.