Students' understanding of “the idea of measurement” can be developed through measurement with standardized - and non-standardized units of measurement. Previous research has identified different difficulties in length measurement, but what does not emerge is how teaching can be designed to meet these difficulties. The purpose of this study is thereby to identify critical aspects, regarding standardized and non-standardized measurement in first grade. The purpose is also to examine how teaching with standardized and non-standard units of measurement can be designed to create conditions for learning. The structure of the study is based on the theory of variation, and the principles of the variation theory is therefore a permeate for the method in this study. Data has been collected through pre-tests, post-tests, worksheets, and lesson designs. The result shows that "distinguish that when measuring with a ruler, the measurement starts from zero" was a critical aspect for the standardized units of measurement. The result also identified "distinguish that the units of measurement must not overlap or have gaps for the measurement to be correct" as a critical aspect for the non-standardized units of measurement. The result shows that when concrete material is used to visualise the units in a measurement, several critical aspects are distinguished. To use rulers with different starting points is proved to be important when teaching with the standardized units of measurement. The post-tests showed that the students who had the lesson with non-standard units of measurement had distinguished most of the critical aspects for both standardized and non-standardized units of measurement after the lesson. The students who focused on the standardized units of measurement, on the other hand, had not discerned any critical aspects for the non-standardized units of measurement after completing the lesson. The result constitutes a conclusion that both standardized and non-standardized units of measurement are needed in teaching, for the students to develop an understanding of “the idea of measurement”.