When two circles intersect there are two possible solutions. However whenever two linkages connect there is only one of these mathematical solutions realised in a physical device. Generally the potential for multiple solutions is designed out, but not always......
Chebyshev's paradoxical mechanism
This is a most intriguing and remarkable mechanism which Artobolevsky 1976 describes as Chebyshev's paradoxical mechanism. It is described as "paradoxical" since one revolution of the crank handle corresponds to two revolutions of the flywheel in the same direction, or to four revolutions in the opposite direction.