Based on BP30, "curve with exactly one self-crossing vs. curve without a self-crossing". This is just a slightly more general version.
Adjacent-numbered pages: BP1227 BP1228 BP1229 BP1230 BP1231  *  BP1233 BP1234 BP1235 BP1236 BP1237
EX6881
stub, notso, traditional
crosscurves [smaller | same | bigger]
Aaron David Fairbanks