In French teaching practices there is currently a lack of consideration for the decimal (base ten) principle of numeration system for whole numbers. This is the reason why we implemented two experiments to strengthen this principle, giving a key role to the use of numeration units (ones, tens, hundreds …). The first one was led in the context of designing a resource for teachers in grade 3, the second one involved training teachers in grades 1 and 2. We analyze how students and teachers take into account numeration units and put them in relation with standard representations of numbers. Both exemplify the complexity of the teaching-learning process of numeration.