For the moment, let us focus on a two-stranded sheet, as in Figure 3.11A: the two component polypeptide chains could come from two distinct polypeptides (an intermolecular sheet) or from two regions of the same contiguous polypeptide (an intramolecular sheet); both types are found in natural proteins. If we imagine a sheet within the plane of this page, we could have both chains running in the same direction, say from C-terminus at the top of the page to N-terminus at the botton. Alternatively, we could have the two chains running in opposite directions with respect to the placement of their Nand C-termini. These two situations describe structures referred to as parallel and antiparallel -pleated sheets, respectively. Again, one finds both types in nature.