  Contact angle hysteresis of droplets will be examined in light of static
friction between liquid drop and solid surface. Unlike frictions in solid-solid
interfaces, pinning forces at contact points (2D) or contact lines (3D) would
be the cause of friction. We will define coefficients of static friction and
relate them with advancing and receding contact angles for the case of
2-dimensional droplets. In our work sessile drops in an inclined plane, and
pendent drops in a slanted ceiling will all be analyzed within a single
framework with the inclination angle as a free parameter. We can then visualize
the gradual change of shapes of a droplet put on a plane as the inclination
angle changes adiabatically to make a complete turn. We also point out that
there could be two distinct stable configurations of pendent droplets for the
same physical conditions, i.e. the bifurcation.
