This study explored the relationship between teachers' content knowledge and their pedagogical skills, and reports on that relationship in the teaching of Assembly Drawing (AD) in a South African context. Given that Engineering Graphics Design (EGD) learners perform poorly in the AD section of the matriculation examination, we need to understand the extent to which this results from the quality of teaching. A case study approach was used to collect qualitative data from 25 purposively selected EGD teachers in the UThukela district of KwaZulu-Natal (KZN). Data was collected via an open-ended questionnaire, focus group interviews, lesson plans, observations, and post observation interviews. Our findings elucidate the relational interplay between teachers' understanding of AD and their teaching of AD. The majority of teachers failed to develop visual, spatial skills in learners. Our findings have implications for continuous teacher professional development.