Non-formal apprenticeship opportunities have a clear record of effectiveness, as evidenced by several research studies. After completing short-term vocational training, apprentices demonstrate the required vocational skills. How is learning constructed in a non-formal learning process? What forms of interaction are developed? What are the determinants of the effectiveness demonstrated by apprentices? This article answers these questions by analysing data from a survey that evaluates the elements of the training context and the level of skills acquired by apprentices. It shows that there is great flexibility in the organisation and techniques of learning, as well as permanent interdependence between apprentices, which facilitates the co-construction of skills.