 Another aspect of informal proof that has been the subject of renewed attention in the recent philosophical literature is the role of diagrams (Giaquinto 2007; Shin &amp; Lemon 2008). What is not in dispute is that proofs—especially in geometry but also in other areas ranging from analysis to group theory—are often accompanied by diagrams. One issue concerns whether such diagrams play an indispensable role in the chain of reasoning leading from the premises of a given proof to its conclusion. Prima facie, there would seem to be three possible situations: