This ethnomathematical research aims to explore, identify, and describe the application of modern formal mathematical concepts, especially the geometry of two-dimensional plane transformation in the ornamental variety of traditional woven fabrics of the Sasak tribe in Sade Traditional Village, Central Lombok. This qualitative research with an ethnographic approach uses research instruments as key instruments. Primary data was obtained through in-depth interviews with senior weavers key informants (Inaq Miakim) as well as directed structural observations on the physical products of finished fabrics, considering that field data collection coincided with the holy month of Ramadan where weaving activities were being paused. The results of the empirical-deductive science analysis prove the integration of an indigenous mathematics system based on multiples of five-strand threads (sesek) units that act as abstract grid coordinates in maintaining pattern congruence. Furthermore, the geometric reconstruction shows that the Subahnale motif applies a combination of bilateral reflection symmetry and constant translation of the hexagonal formation of a rhombus; the Rangrang motif is dominated by congruent rhombus polygons through diagonal and horizontal translation; and the Bracket bulen motif applies the principle of the Cartesian double reflection axis (Quadrant I, II, III, and IV) to lock the position of the central ornament. These findings have implications as an alternative to culture-based geometry learning contexts through the Contextual Teaching and Learning (CTL) approach in secondary schools.