Sub-singularities for shaping thin sheets

The principles behind the sharp, singular structures in a crumpled sheet are well understood. Here we discuss more general ways of exploiting such sharp structures to control the shape of a sheet by deforming or forcing it elsewhere. Often, the induced shape leads to further sharp structures -- ``sub-singularities". Though weaker and softer than the primary singularities, they nevertheless provide robust ways of shaping a sheet. In simple cases we understand the reason for these and their strength. This paper surveys a broad range of other sub-structure phenomena and reports recent progress in understanding some of them.