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This research is a descriptive study with a qualitative approach. The purpose of this study was to determine the ability to understand mathematical concepts of vocational students at the level of interpolation and extrapolation in arithmetic sequence material. Data collection was carried out on students of class XI vocational high school with the ability to understand the mathematical concepts of interpolation and extrapolation levels. Data collection techniques used in this study are tests, interviews and documentation. Data analysis techniques in this study are data reduction, data display and conclusion drawing / verification. Data validity uses technical triangulation. The results of the analysis show that students with a level of understanding of the concept of interpolation mathematics are only able to fulfill two indicators, namely giving examples or counter examples (opposite examples) of the concepts studied and presenting concepts in various forms of mathematical representations (tables, graphs, diagrams, sketches, mathematical models or other ways). students with a level of understanding of extrapolation mathematical concepts are able to fulfill five indicators, namely restating the concepts that have been studied, identifying the characteristics of operations or concepts, giving examples or examples of counter (versus examples) of the concepts studied, presenting concepts in various forms of mathematical representations ( tables, graphs, diagrams, sketches, mathematical models or other methods), and link various concepts in mathematics and outside mathematics. Keywords: Analysis, Mathematical Conceptual Understanding, Etnomatematics