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
M. M. Bongard, Pattern Recognition, Spartan Books, 1970, p. 223.
BP1232 is almost exactly the same, but allowing for multiple curves and multiple self-crossings.
Adjacent-numbered pages: BP25 BP26 BP27 BP28 BP29  *  BP31 BP32 BP33 BP34 BP35
nice, finished, traditional, bongard
crosscurve [smaller | same | bigger] zoom in right (curve_or_outline)
Mikhail M. Bongard