Investigated was the question of whether understanding is explained by means of concepts referring to tasks, being domain-specific for certain subject matter areas, or is it explained by concepts which extend across tasks and are not domain-specific? Also explored were: (1) which explanatory concepts are present, that is, which steps and conditions of understanding are thematized in the explanations? and (2) do explanatory concepts differentiate according to success and failure in understanding or by sex? A two-dimensional table was constructed for coding explanations of 20 male mathematics teachers in grades 5 and 6 of schools in West Germany. Results indicated a marked predominance of explanations referring to domain-specific tasks as compared to explanations extending across tasks. The order of frequency of steps and conditions of understanding are noted, and sex differences in a few categories were found. The discussion focuses on: (1) reference to the task and type of explanation; and (2) obtaining explanations referring to task: problems and opportunities. (MNS)