Handling mods

Panhard bars control the side to side movement of an axle. In a leaf spring car, that job is pretty much covered by the springs, though a Panhard could be added. Usually connected from one frame rail to the opposite side on the axle. Remember it will want to travel in an arc since one side is "fixed" (theoretically). The longer the bar, the less radical the arc, also, the closer to level, the better. Panhard rod - Wikipedia
A Watts-Link can do a more accurate job by keeping the axle perfectly centered because of no arc, but is more complex. Watt's linkage - Wikipedia
In a Three-Link, or a straight Four-Link car it is needed because the coil springs can't perform that job.
In a Triangulated Four-Link, the upper links are angled in toward the center of the vehicle, preventing side-side movement.