- Xabier Iriarte
This linkage by Richard Roberts represents the most general "four bar" linkage. The point P is on the middle link BC, but offset from it. Here the 4th bar is the link AD, which is also the fixed base. Notice that with these particular measurements the point P is very close indeed to the straight line between AD.
The complete locus is rather complex, since from a mathematical point of view the linkage could be fliped down, and the point P could be independently flipped up, giving four branches to the locus.