with only pin joints requires more than four links.
At least six links and seven pin joints are needed to generate an exact straight line with a
pure revolute-jointed linkage, i.e., a Watt's or Stephenson's sixbar. A geared fivebar
mechanism, with a gear ratio of -1 and a phase angle of 1t radians, will generate an exact
straight line at the joint between links 3 and 4. But this linkage is merely a transformed
Watt's sixbar obtained by replacing one binary link with a higher joint in the form of a
gear pair. This geared fivebar's straight-line motion can be seen by reading the file
STRAIGHT.5BRinto program FIVEBAR,calculating and animating the linkage.