Readers of "Teaching History" will be familiar with the benefits and difficulties of cross-curricular planning, and the pages of this journal have often carried analysis of successful collaborations with the English department, or music, or geography. Harry Fletcher-Wood describes in this article a collaboration involving maths, providing for us the fruits of both his own labour and that of his mathematical counterpart Danny Brown. Fletcher-Wood's article sets out a compelling rationale for this study, and provides some practical ideas of what, exactly, to do. Even more interesting though, is the reflection which forms the second half of this article. For--like any other project in its first year--this collaboration did not entirely work. Fletcher-Wood takes us honestly through his analysis of what went wrong, and how he fixed it--and of what went wrong next, and how he fixed that. The resulting enquiry is far broader than was the original, taking in far more subjects but with history more firmly at the centre. Fletcher-Wood also raises an interesting question of how to ensure, in a small department (in this case, of one) that history remains at the centre, even of a cross-curricular enquiry. Finally, if you'd even wondered why crossing bridges in Germany is so important, or how to draw a circle and a triangle (and why it matters) then this article can provide some answers.