This article presents four different teaching experiences concerning decimal multiplication in sixth grade (11 years), all carried out in similar circumstances. The analysis studies the preparation which went into the observed lessons and the results in the classroom. It uses a double approach: the first one applies didactics of mathematics, the second one borrows a few elementary theories from the field of ergonomic psychology. The studied teaching experiences all demonstrate certain restraints which explain, on a macro level, the similarities of the four experiences. There is, however, beyond these restraints, a certain amount of individual decision-making that the observed teachers were using. This factor explains, on a micro level, the pedagogical diversity seen in these four classes.