Figure 2.

For a fixed base position of the metacarpal phalangeal joint M, there are infinitely many solutions of ${\text{θ}}_{MCP}$, ${\text{θ}}_{PIP}$ and ${\text{θ}}_{DIP}$, or in other words, P and D, to place the fingertip T into a particular position, in addition to the knowledge of the lengths of the phalangeal bones ($l_1$, $l_2$ and $l_3$).