All examples feature a white circle and a black circle. The answer refers to the slope of the line connecting the two circles.
The coloring of the circles is irrelevant.
Adjacent-numbered pages: BP101 BP102 BP103 BP104 BP105  *  BP107 BP108 BP109 BP110 BP111
nice, noisy, dual, handed, rotate, orderedpair, traditional
Douglas R. Hofstadter