The relationship between learning and context has long been an issue of interest and concern in the field of adult mathematics education. In particular, the questions of whether and how learning can be transferred from one context to another, remains a focus of researchers (Lave 1988, Lerman 1999, Evans 2000, Carraher and Scheliemann 2002). In this paper we look at the mathematical understanding of a group of apprentice ironworkers working on a construction task, and explore the flexible nature of their understanding. We consider the ways in which they are able to use quite formal mathematical ideas and operations and make sense of these within the specific context of their trade. Then we discuss how this process is more than one of simple transfer.