Double-quadrant encoding of orientations. The relative orientation of any two SSEs is encoded as a two-character string with the two quadrant schemes shown, which differ in orientation by 45° in order to prevent small variations resulting in completely different encodings. The first quadrant encoding is labelled P, O, L, R for parallel, anti-parallel, crossing-left, and crossing-right, respectively, and the second arbitrarily E, D, S, T. For example, two SSEs that are anti-parallel would be encoded as either OS or OT.
Stivala et al. BMC Bioinformatics 2010 11:446 doi:10.1186/1471-2105-11-446