The method of joints finds truss-member forces by applying force equilibrium to one pin joint at a time.

Because an ideal truss joint cannot transmit a moment, each isolated joint is governed by

$$\sum F_x=0,\qquad \sum F_y=0.$$

Start from the whole truss

First determine the external support reactions by treating the entire truss as one rigid body.

Isolate a solvable joint

Choose a joint with no more than two unknown member forces. Draw the forces exerted by all connected members along their member axes.

It is convenient to assume each unknown member is initially in tension, so its force arrow points away from the joint. A negative result then indicates compression.

Move through the structure

Once one or more member forces are known, neighboring joints acquire fewer unknowns and can be solved in turn.

The method is especially useful when the forces in most or all members are required.